feat: add graphify plugin and documentation

- Introduced a new graphify OpenCode plugin to remind users about the knowledge graph before executing bash commands.
- Added AGENTS.md for agent guidance, including development commands, environment setup, Docker deployment, code structure, conventions, and testing instructions.
- Created opencode.json to configure the graphify plugin and superpowers.
- Updated tests to improve type safety and added missing imports in test files.
- Added .graphify_uncached.txt to track relevant files for graphify.

.gitea/workflows/build.yml

@@ -20,6 +20,18 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+          cache: 'npm'
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Run tests
+        run: npm run test
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 

.gitignore

@@ -53,3 +53,4 @@ data/documents/
 
 .vscode/
 .react-router
+.continue

.graphify_uncached.txt

@@ -0,0 +1,21 @@
+AGENTS.md
+copilot-instructions.md
+README.md
+CLAUDE.md
+IMPROVEMENTS_SUMMARY.md
+INTEGRATION_EXAMPLE.md
+tests/README.md
+app/lib/ERROR_LOGGING_GUIDE.md
+graphify-out/GRAPH_REPORT.md
+data/documents/cmmoo4v5p0000ykou0bcivjsn/migr-ein-cmn4ogjc20007dfmmex6engdh-1777488241127.pdf
+data/documents/cmmoo4v5p0000ykou0bcivjsn/migr-ein-cmn4ogjc20007dfmmex6engdh-1777488343334.pdf
+data/documents/cmootxs7r0000nvgjmhgbanvx/demo-invoice-1-1777790365605.pdf
+data/documents/cmootxs7r0000nvgjmhgbanvx/cmopen7qx000s2mzpnq44y0rb-1777790640988.pdf
+data/documents/cmootxs7r0000nvgjmhgbanvx/cmopejaq8000f2mzpnho6t72z-1777790461825.pdf
+public/file.svg
+public/window.svg
+public/globe.svg
+public/next.svg
+public/vercel.svg
+coverage/favicon.png
+coverage/sort-arrow-sprite.png

.opencode/plugins/graphify.js

@@ -0,0 +1,22 @@
+// graphify OpenCode plugin
+// Injects a knowledge graph reminder before bash tool calls when the graph exists.
+import { existsSync } from "fs";
+import { join } from "path";
+
+export const GraphifyPlugin = async ({ directory }) => {
+  let reminded = false;
+
+  return {
+    "tool.execute.before": async (input, output) => {
+      if (reminded) return;
+      if (!existsSync(join(directory, "graphify-out", "graph.json"))) return;
+
+      if (input.tool === "bash") {
+        output.args.command =
+          'echo "[graphify] Knowledge graph available. Read graphify-out/GRAPH_REPORT.md for god nodes and architecture context before searching files." && ' +
+          output.args.command;
+        reminded = true;
+      }
+    },
+  };
+};

AGENTS.md

@@ -0,0 +1,53 @@
+# Annas Rechnungsmanager Agent Guidance
+
+## Development Commands
+- Install: `npm install`
+- Dev server: `npm run dev` (Vite on http://localhost:5173)
+- Dev with DB: `npm run devfull` (starts docker-compose DB first)
+- Typecheck: `npm run typecheck` (react-router typegen + tsc)
+- Lint: `npm run lint` (eslint)
+- Test: `npm run test` (vitest run)
+- DB migrate: `npm run db:migrate` (prisma migrate dev)
+- DB seed: `npm run db:seed` (loads demo data)
+- DB studio: `npm run db:studio` (Prisma GUI)
+- Admin setup: `npm run setup-admin` (sets ADMIN_PASSWORD env var or prompts)
+- Reset password: `npm run reset-password -- --username <user> --password <pass>`
+- Build: `npm run build`
+- Start prod: `npm run start`
+
+## Environment
+- `.env` required: DATABASE_URL and AUTH_SECRET
+- Dev: http://localhost:5173
+- Docker: http://localhost:3000 (first start requires ADMIN_PASSWORD)
+
+## Docker Deployment
+- First start: `docker build -t annasrechnungsmanager:latest .` then `docker compose up -d` with ADMIN_PASSWORD set
+- Subsequent: `docker build -t annasrechnungsmanager:latest .` then `docker compose up -d --no-deps app`
+- Admin user created/updated on first start with ADMIN_PASSWORD
+
+## Code Structure
+- `app/routes/` - file-based routing (React Router v7)
+- `app/routes/api.*` - REST API endpoints (loader/action)
+- `app/lib/` - data access, Prisma, tax, utils
+- `prisma/schema.prisma` - database schema
+- `scripts/` - setup-admin, reset-password
+
+## Conventions
+- TypeScript strict; use unknown/guard checks for external data
+- UI: Tailwind v4 + shadcn/ui
+- Error handling: Remix redirect, json, badRequest
+- API contracts in `app/routes/api.*` must remain backward compatible
+- Document business logic for complex changes (taxes, invoice codes, UStG §14)
+
+## Testing
+- Vitest: `npm run test` (run), `npm run test:watch` (watch)
+- No additional services required for unit tests
+
+## graphify
+
+This project has a graphify knowledge graph at graphify-out/.
+
+Rules:
+- Before answering architecture or codebase questions, read graphify-out/GRAPH_REPORT.md for god nodes and community structure
+- If graphify-out/wiki/index.md exists, navigate it instead of reading raw files
+- After modifying code files in this session, run `python3 -c "from graphify.watch import _rebuild_code; from pathlib import Path; _rebuild_code(Path('.'))"` to keep the graph current

opencode.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "https://opencode.ai/config.json",
+  "plugin": [
+    ".opencode/plugins/graphify.js",
+    "superpowers@git+https://github.com/obra/superpowers.git"
+  ]
+}

package.json

@@ -10,6 +10,10 @@
     "start": "react-router-serve ./build/server/index.js",
     "typecheck": "react-router typegen && tsc",
     "lint": "eslint",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "test:ui": "vitest --ui",
+    "test:coverage": "vitest run --coverage",
     "db:migrate": "prisma migrate dev",
     "db:seed": "npx ts-node --compiler-options '{\"module\":\"CommonJS\"}' prisma/seed.ts",
     "db:studio": "prisma studio",
@@ -54,15 +58,21 @@
   "devDependencies": {
     "@react-router/dev": "^7.13.1",
     "@tailwindcss/postcss": "^4",
+    "@testing-library/jest-dom": "^6.9.1",
+    "@testing-library/react": "^16.3.2",
     "@types/node": "^20",
     "@types/node-cron": "^3.0.11",
     "@types/react": "^19",
     "@types/react-dom": "^19",
     "@vitejs/plugin-react": "^4",
+    "@vitest/coverage-v8": "^4.1.5",
+    "@vitest/ui": "^4.1.5",
     "eslint": "^9",
+    "jsdom": "^29.1.1",
     "tailwindcss": "^4",
     "typescript": "^5",
     "vite": "^6",
-    "vite-tsconfig-paths": "^5"
+    "vite-tsconfig-paths": "^5",
+    "vitest": "^4.1.5"
   }
 }

tests/README.md

@@ -0,0 +1,67 @@
+# Tests - Annas RechnungsManager
+
+Vitest-basiertes Test-Framework für kritische Geschäftslogik.
+
+## Setup
+
+```bash
+# Install dependencies (already done)
+npm install
+
+# Run tests
+npm run test           # Single run
+npm run test:watch     # Watch mode
+npm run test:ui        # Browser UI
+npm run test:coverage  # With coverage report
+```
+
+## Test Structure
+
+```
+tests/
+├── lib/
+│   ├── tax.test.ts          # Steuerberechnungen (§14 UStG)
+│   └── schemas.test.ts      # Zod-Validierung (Input-Sicherheit)
+└── README.md
+```
+
+## Coverage
+
+- ✅ **tax.ts** - German tax calculations (19%, 7%, Kleinunternehmer)
+- ✅ **schemas.ts** - Input validation (Zod schemas)
+- ⚠️ **invoice-number.server.ts** - Requires Prisma mocking (TODO)
+
+## Critical Test Areas
+
+1. **Tax Calculations** - Must be correct per German tax law
+2. **Input Validation** - Prevent invalid data in DB
+3. **Invoice Logic** - §14 UStG compliance
+4. **Buchhaltung** - Double-entry bookkeeping accuracy
+
+## Running Specific Tests
+
+```bash
+# Run only tax tests
+npx vitest run tests/lib/tax.test.ts
+
+# Run with coverage
+npm run test:coverage
+# Opens: ./coverage/index.html
+```
+
+## Writing New Tests
+
+```typescript
+import { describe, it, expect } from "vitest";
+import { myFunction } from "@/lib/my-module";
+
+describe("myModule", () => {
+  it("should do something", () => {
+    expect(myFunction()).toBe(expected);
+  });
+});
+```
+
+## CI Integration
+
+Tests laufen automatisch in der Gitea Actions Pipeline (`.gitea/workflows/build.yml`).

tests/components/invoice-status-badge.test.tsx

@@ -0,0 +1,39 @@
+import { describe, it, expect } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { InvoiceStatusBadge } from "@/components/invoice/invoice-status-badge";
+import { InvoiceStatus } from "@prisma/client";
+
+describe("InvoiceStatusBadge", () => {
+  it("should render DRAFT status correctly", () => {
+    render(<InvoiceStatusBadge status={InvoiceStatus.DRAFT} />);
+    expect(screen.getByText("Entwurf")).toBeInTheDocument();
+  });
+
+  it("should render SENT status correctly", () => {
+    render(<InvoiceStatusBadge status={InvoiceStatus.SENT} />);
+    expect(screen.getByText("Versendet")).toBeInTheDocument();
+  });
+
+  it("should render PAID status correctly", () => {
+    render(<InvoiceStatusBadge status={InvoiceStatus.PAID} />);
+    expect(screen.getByText("Bezahlt")).toBeInTheDocument();
+  });
+
+  it("should render CANCELLED status correctly", () => {
+    render(<InvoiceStatusBadge status={InvoiceStatus.CANCELLED} />);
+    expect(screen.getByText("Storniert")).toBeInTheDocument();
+  });
+
+  it("should render DELETED status correctly", () => {
+    render(<InvoiceStatusBadge status={InvoiceStatus.DELETED} />);
+    expect(screen.getByText("Gelöscht")).toBeInTheDocument();
+  });
+
+  it("should have proper badge structure", () => {
+    render(<InvoiceStatusBadge status={InvoiceStatus.PAID} />);
+    const badge = screen.getByText("Bezahlt");
+    expect(badge).toBeInTheDocument();
+    // Badge should be a div element
+    expect(badge.tagName).toBe("DIV");
+  });
+});

tests/integration/api-simple.test.ts

@@ -0,0 +1,198 @@
+import { describe, it, expect } from "vitest";
+
+/**
+ * Simple Integration Tests
+ * 
+ * These tests verify that the API logic works correctly
+ * without requiring a full database connection.
+ */
+
+describe("API Integration (Simple)", () => {
+  describe("Company API Logic", () => {
+    it("should validate company creation data", async () => {
+      const { companySchema } = await import("@/lib/schemas");
+      
+      const validCompany = {
+        name: "Test GmbH",
+        address: "Hauptstraße 1",
+        zip: "12345",
+        city: "Berlin",
+        country: "DE",
+        invoicePrefix: "RE",
+        kleinunternehmer: false,
+        taxId: null,
+        vatId: null,
+        bankIban: null,
+        bankBic: "",
+      };
+
+      const result = companySchema.safeParse(validCompany);
+      expect(result.success).toBe(true);
+    });
+
+    it("should reject invalid company data", async () => {
+      const { companySchema } = await import("@/lib/schemas");
+      
+      const invalidCompany = {
+        name: "", // Invalid: empty name
+        address: "Test St. 1",
+        zip: "12345",
+        city: "Berlin",
+      };
+
+      const result = companySchema.safeParse(invalidCompany);
+      expect(result.success).toBe(false);
+    });
+  });
+
+  describe("Invoice API Logic", () => {
+    it("should validate invoice creation data", async () => {
+      const { invoiceSchema } = await import("@/lib/schemas");
+      
+      const validInvoice = {
+        companyId: "company-123",
+        customerId: "customer-456",
+        issueDate: "2026-05-08",
+        dueDate: "2026-06-08",
+        kleinunternehmer: false,
+        items: [
+          {
+            position: 1,
+            description: "Web Development",
+            quantity: 10,
+            unit: "Stunden",
+            unitPrice: 100,
+            taxRate: 19,
+            netAmount: 1000,
+            taxAmount: 190,
+            grossAmount: 1190,
+          },
+        ],
+        netTotal: 1000,
+        taxTotal: 190,
+        grossTotal: 1190,
+      };
+
+      const result = invoiceSchema.safeParse(validInvoice);
+      expect(result.success).toBe(true);
+    });
+
+    it("should reject invoice with no items", async () => {
+      const { invoiceSchema } = await import("@/lib/schemas");
+      
+      const invalidInvoice = {
+        companyId: "company-123",
+        customerId: "customer-456",
+        issueDate: "2026-05-08",
+        dueDate: "2026-06-08",
+        items: [], // Invalid: no items
+        netTotal: 0,
+        taxTotal: 0,
+        grossTotal: 0,
+      };
+
+      const result = invoiceSchema.safeParse(invalidInvoice);
+      expect(result.success).toBe(false);
+    });
+  });
+
+  describe("Customer API Logic", () => {
+    it("should validate customer data", async () => {
+      const { customerSchema } = await import("@/lib/schemas");
+      
+      const validCustomer = {
+        companyId: "company-123",
+        name: "Max Mustermann",
+        address: "Musterstraße 1",
+        zip: "12345",
+        city: "Berlin",
+        country: "DE",
+      };
+
+      const result = customerSchema.safeParse(validCustomer);
+      expect(result.success).toBe(true);
+    });
+
+    it("should require companyId", async () => {
+      const { customerSchema } = await import("@/lib/schemas");
+      
+      const invalidCustomer = {
+        name: "Max Mustermann",
+        address: "Musterstraße 1",
+        zip: "12345",
+        city: "Berlin",
+      };
+
+      const result = customerSchema.safeParse(invalidCustomer);
+      expect(result.success).toBe(false);
+    });
+  });
+
+  describe("Tax Calculation Integration", () => {
+    it("should calculate invoice totals from items", async () => {
+      const { calcItemAmounts, calcInvoiceTotals } = await import("@/lib/tax");
+      
+      const items = [
+        {
+          quantity: 10,
+          unitPrice: 100,
+          taxRate: 19,
+        },
+        {
+          quantity: 5,
+          unitPrice: 200,
+          taxRate: 7,
+        },
+      ].map((item, idx) => ({
+        position: idx + 1,
+        description: `Service ${idx + 1}`,
+        ...item,
+        ...calcItemAmounts(item.quantity, item.unitPrice, item.taxRate),
+      }));
+
+      const totals = calcInvoiceTotals(items);
+
+      expect(totals.netTotal).toBe(2000); // 1000 + 1000
+      expect(totals.taxTotal).toBe(260); // 190 + 70 (5*200*7% = 70)
+      expect(totals.grossTotal).toBe(2260); // 1190 + 1070
+    });
+
+    it("should handle Kleinunternehmer calculation", async () => {
+      const { calcItemAmountsKleinunternehmer, calcInvoiceTotals } = await import("@/lib/tax");
+      
+      const items = [
+        {
+          quantity: 10,
+          unitPrice: 100,
+        },
+      ].map((item, idx) => ({
+        position: idx + 1,
+        description: "Service",
+        ...item,
+        ...calcItemAmountsKleinunternehmer(item.quantity, item.unitPrice),
+        taxRate: 0,
+      }));
+
+      const totals = calcInvoiceTotals(items);
+
+      expect(totals.netTotal).toBe(1000);
+      expect(totals.taxTotal).toBe(0);
+      expect(totals.grossTotal).toBe(1000);
+    });
+  });
+
+  describe("Authentication Logic", () => {
+    it("should identify unauthorized requests", () => {
+      const user = null;
+      const isAuthenticated = user !== null;
+      expect(isAuthenticated).toBe(false);
+    });
+
+    it("should identify authorized requests", () => {
+      const user = { id: "user-123", role: "ADMIN" };
+      const isAuthenticated = user !== null;
+      expect(isAuthenticated).toBe(true);
+      expect(user.role).toBe("ADMIN");
+    });
+  });
+});

tests/integration/api.test.ts

@@ -0,0 +1,190 @@
+import { describe, it, expect, beforeAll, afterAll, beforeEach } from "vitest";
+import { testPrisma, setupTestDatabase, cleanupTestDatabase, createTestUser, createTestCompany, createTestCustomer } from "./setup";
+import bcrypt from "bcryptjs";
+
+/**
+ * Integration Tests for API Routes
+ * 
+ * These tests require a test database.
+ * They will skip gracefully if the database is not available.
+ */
+
+describe("API Integration Tests (Database Required)", () => {
+  let testUser: any;
+  let testCompany: any;
+  let testCustomer: any;
+  let dbAvailable = false;
+
+  // Setup before all tests
+  beforeAll(async () => {
+    dbAvailable = await setupTestDatabase();
+    
+    if (!dbAvailable) {
+      console.warn("Skipping database integration tests - no test database available");
+      return;
+    }
+    
+    // Create test user
+    testUser = await createTestUser("test@example.com", "testuser");
+    
+    // Create test company
+    testCompany = await createTestCompany(testUser.id, "Integration Test GmbH");
+    
+    // Create test customer
+    testCustomer = await createTestCustomer(testCompany.id, "Integration Test Customer");
+  });
+
+  // Cleanup after all tests
+  afterAll(async () => {
+    if (!dbAvailable) return;
+    await cleanupTestDatabase();
+    await testPrisma.$disconnect();
+  });
+
+  // Clean data between tests (but keep user/company/customer)
+  beforeEach(async () => {
+    if (!dbAvailable) return;
+    
+    // Delete invoices and related items
+    await testPrisma.invoiceItem.deleteMany({
+      where: { invoice: { companyId: testCompany.id } },
+    });
+    await testPrisma.invoice.deleteMany({
+      where: { companyId: testCompany.id },
+    });
+  });
+
+  // Helper to skip tests if no database
+  const dbTest = (name: string, fn: () => Promise<void>) => {
+    it(name, async () => {
+      if (!dbAvailable) {
+        console.warn(`Skipping "${name}" - no test database`);
+        return;
+      }
+      await fn();
+    });
+  };
+
+  describe("Companies API", () => {
+    dbTest("should list companies for a user", async () => {
+      const companies = await testPrisma.company.findMany({
+        where: { userId: testUser.id },
+      });
+
+      expect(companies).toBeDefined();
+      expect(companies.length).toBeGreaterThan(0);
+      expect(companies[0].name).toBe("Integration Test GmbH");
+    });
+
+    dbTest("should create a new company", async () => {
+      const newCompanyData = {
+        name: "New Test Company",
+        address: "New Street 1",
+        zip: "99999",
+        city: "Munich",
+        country: "DE",
+      };
+
+      const company = await testPrisma.company.create({
+        data: {
+          ...newCompanyData,
+          userId: testUser.id,
+        },
+      });
+
+      expect(company).toBeDefined();
+      expect(company.name).toBe("New Test Company");
+      expect(company.userId).toBe(testUser.id);
+    });
+  });
+
+  describe("Invoices API", () => {
+    dbTest("should create a new invoice", async () => {
+      const { calcItemAmounts, calcInvoiceTotals } = await import("@/lib/tax");
+      
+      const invoiceData = {
+        companyId: testCompany.id,
+        customerId: testCustomer.id,
+        issueDate: new Date(),
+        dueDate: new Date(Date.now() + 30 * 24 * 60 * 60 * 1000),
+        kleinunternehmer: false,
+        items: [
+          {
+            position: 1,
+            description: "Test Service",
+            quantity: 10,
+            unit: "Stunden",
+            unitPrice: 100,
+            taxRate: 19,
+            netAmount: 1000,
+            taxAmount: 190,
+            grossAmount: 1190,
+          },
+        ],
+      };
+
+      // Recalculate server-side (as the API does)
+      const recalculatedItems = invoiceData.items.map(item => ({
+        ...item,
+        ...calcItemAmounts(item.quantity, item.unitPrice, item.taxRate),
+      }));
+      const totals = calcInvoiceTotals(recalculatedItems);
+
+      const invoice = await testPrisma.invoice.create({
+        data: {
+          companyId: invoiceData.companyId,
+          customerId: invoiceData.customerId,
+          issueDate: invoiceData.issueDate,
+          dueDate: invoiceData.dueDate,
+          status: "DRAFT",
+          kleinunternehmer: invoiceData.kleinunternehmer,
+          netTotal: totals.netTotal,
+          taxTotal: totals.taxTotal,
+          grossTotal: totals.grossTotal,
+          items: {
+            create: recalculatedItems.map((item, idx) => ({
+              position: idx + 1,
+              description: item.description,
+              quantity: item.quantity,
+              unit: item.unit,
+              unitPrice: item.unitPrice,
+              taxRate: item.taxRate,
+              netAmount: item.netAmount,
+              taxAmount: item.taxAmount,
+              grossAmount: item.grossAmount,
+            })),
+          },
+        },
+        include: { items: true, customer: true },
+      });
+
+      expect(invoice).toBeDefined();
+      expect(invoice.status).toBe("DRAFT");
+      expect(invoice.items).toHaveLength(1);
+      expect(invoice.grossTotal).toBe(1190);
+    });
+  });
+
+  describe("Buchungen API", () => {
+    dbTest("should create a new Buchung", async () => {
+      const buchungData = {
+        companyId: testCompany.id,
+        date: new Date(),
+        account: "KASSE" as const,
+        type: "EINLAGE" as const,
+        amount: 1000,
+        description: "Initial investment",
+        isBusinessRecord: false,
+      };
+
+      const buchung = await testPrisma.buchung.create({
+        data: buchungData,
+      });
+
+      expect(buchung).toBeDefined();
+      expect(buchung.account).toBe("KASSE");
+      expect(buchung.type).toBe("EINLAGE");
+      expect(buchung.amount.toNumber()).toBe(1000);
+    });
+  });
+});

tests/integration/setup.ts

@@ -0,0 +1,128 @@
+import { PrismaClient } from "@prisma/client";
+import { execSync } from "child_process";
+
+/**
+ * Integration Test Setup
+ * Uses a separate test database to avoid polluting development data
+ */
+
+const TEST_DATABASE_URL = process.env.TEST_DATABASE_URL || 
+  "mysql://annas_user:annas_password@localhost:3306/annas_rechnungen_test";
+
+export const testPrisma = new PrismaClient({
+  datasources: {
+    db: {
+      url: TEST_DATABASE_URL,
+    },
+  },
+});
+
+/**
+ * Setup test database: push schema and seed if needed
+ * Returns true if successful, false if database unavailable
+ */
+export async function setupTestDatabase(): Promise<boolean> {
+  console.log("Setting up test database...");
+  
+  try {
+    // Push schema to test database
+    execSync(`npx prisma db push --force-reset`, {
+      env: {
+        ...process.env,
+        DATABASE_URL: TEST_DATABASE_URL,
+      },
+      stdio: "inherit",
+    });
+    
+    console.log("Test database ready");
+    return true;
+  } catch (error) {
+    console.warn("Test database not available:", error);
+    return false;
+  }
+}
+
+/**
+ * Clean up test database
+ */
+export async function cleanupTestDatabase() {
+  // Delete all data in correct order (respecting foreign keys)
+  const tables = [
+    "audit_logs",
+    "invoice_items",
+    "invoices",
+    "buchung_kategorien",
+    "buchungen",
+    "anlagegueter",
+    "services",
+    "customers",
+    "companies",
+    "users",
+  ];
+
+  for (const table of tables) {
+    await testPrisma.$executeRawUnsafe(`DELETE FROM ${table}`);
+  }
+}
+
+/**
+ * Create a test user
+ */
+export async function createTestUser(email: string, username: string) {
+  const bcrypt = await import("bcryptjs");
+  const passwordHash = await bcrypt.default.hash("test1234", 10);
+
+  return testPrisma.user.create({
+    data: {
+      email,
+      username,
+      name: "Test User",
+      passwordHash,
+      role: "ADMIN",
+    },
+  });
+}
+
+/**
+ * Create a test company for a user
+ */
+export async function createTestCompany(userId: string, name: string = "Test GmbH") {
+  return testPrisma.company.create({
+    data: {
+      name,
+      address: "Teststraße 1",
+      zip: "12345",
+      city: "Berlin",
+      country: "DE",
+      userId,
+    },
+  });
+}
+
+/**
+ * Create a test customer for a company
+ */
+export async function createTestCustomer(companyId: string, name: string = "Test Customer") {
+  return testPrisma.customer.create({
+    data: {
+      name,
+      address: "Kundenstraße 1",
+      zip: "54321",
+      city: "Hamburg",
+      country: "DE",
+      companyId,
+    },
+  });
+}
+
+/**
+ * Generate auth cookie for test requests
+ * (Simplified - in real scenario, you'd create a session)
+ */
+export function getAuthHeaders(userId: string) {
+  // For integration tests, we might mock the session or use a test session
+  return {
+    "Content-Type": "application/json",
+    "X-Test-User-Id": userId, // Custom header for test auth bypass
+  };
+}

tests/lib/afa.test.ts

@@ -0,0 +1,159 @@
+import { describe, it, expect } from "vitest";
+import {
+  jahresAfa,
+  erwerbsjahrAfa,
+  afaFuerJahr,
+  kumulierteAfa,
+  buchwert,
+  assetStatus,
+  AnlagegutRaw,
+} from "@/lib/afa";
+
+describe("afa.ts - Asset Depreciation (§7 EStG)", () => {
+  const sampleAsset: AnlagegutRaw = {
+    anschaffungskosten: 12000,
+    nutzungsdauerJahre: 3,
+    restwert: 0,
+    anschaffungsdatum: "2024-03-15", // March 15, 2024
+    aktiv: true,
+  };
+
+  describe("jahresAfa", () => {
+    it("should calculate full annual depreciation", () => {
+      // 12000 over 3 years = 4000 per year
+      expect(jahresAfa(12000, 0, 3)).toBe(4000);
+    });
+
+    it("should handle residual value (restwert)", () => {
+      // 12000 - 2000 restwert = 10000 over 3 years
+      expect(jahresAfa(12000, 2000, 3)).toBe(3333.33);
+    });
+
+    it("should handle different depreciation periods", () => {
+      // 24000 over 8 years = 3000 per year
+      expect(jahresAfa(24000, 0, 8)).toBe(3000);
+    });
+  });
+
+  describe("erwerbsjahrAfa", () => {
+    it("should calculate pro-rata depreciation in acquisition year", () => {
+      // Acquired March 15 = 10 months remaining (Mar-Dec, inclusive) = 10/12
+      // 4000 * (10/12) = 3333.33
+      const acqDate = new Date("2024-03-15");
+      const result = erwerbsjahrAfa(12000, 0, 3, acqDate);
+      expect(result).toBe(3333.33);
+    });
+
+    it("should calculate full year if acquired in January", () => {
+      const acqDate = new Date("2024-01-01");
+      const result = erwerbsjahrAfa(12000, 0, 3, acqDate);
+      expect(result).toBe(4000);
+    });
+
+    it("should calculate 1/12 if acquired in December", () => {
+      const acqDate = new Date("2024-12-01");
+      const result = erwerbsjahrAfa(12000, 0, 3, acqDate);
+      expect(result).toBe(333.33);
+    });
+  });
+
+  describe("afaFuerJahr", () => {
+    it("should return 0 for years before acquisition", () => {
+      expect(afaFuerJahr(sampleAsset, 2023)).toBe(0);
+    });
+
+    it("should return pro-rata for acquisition year", () => {
+      // 2024 acquisition, 10 months = 3333.33
+      const result = afaFuerJahr(sampleAsset, 2024);
+      expect(result).toBe(3333.33);
+    });
+
+    it("should return full annual amount for middle years", () => {
+      expect(afaFuerJahr(sampleAsset, 2025)).toBe(4000);
+      expect(afaFuerJahr(sampleAsset, 2026)).toBe(4000);
+    });
+
+    it("should return 0 after full depreciation period", () => {
+      // 3 years: 2024, 2025, 2026 -> after 2026 = 0
+      expect(afaFuerJahr(sampleAsset, 2027)).toBe(0);
+    });
+
+    it("should handle asset with residual value", () => {
+      const assetWithRestwert: AnlagegutRaw = {
+        anschaffungskosten: 5000,
+        nutzungsdauerJahre: 5,
+        restwert: 500,
+        anschaffungsdatum: "2024-01-01",
+        aktiv: true,
+      };
+      // (5000-500) / 5 = 900 per year
+      expect(afaFuerJahr(assetWithRestwert, 2024)).toBe(900);
+      expect(afaFuerJahr(assetWithRestwert, 2028)).toBe(900);
+      expect(afaFuerJahr(assetWithRestwert, 2029)).toBe(0);
+    });
+  });
+
+  describe("kumulierteAfa", () => {
+    it("should calculate cumulative depreciation correctly", () => {
+      // 2024: 3333.33, 2025: 4000, 2026: 4000 = 11333.33
+      expect(kumulierteAfa(sampleAsset, 2026)).toBe(11333.33);
+    });
+
+    it("should return 0 for year before acquisition", () => {
+      expect(kumulierteAfa(sampleAsset, 2023)).toBe(0);
+    });
+
+    it("should return full depreciation after end of period", () => {
+      // After 3 years: 3333.33 + 4000 + 4000 = 11333.33
+      // But asset cost is 12000, so 12000 - 11333.33 = 666.67 remaining
+      const result = kumulierteAfa(sampleAsset, 2027);
+      expect(result).toBe(11333.33);
+    });
+  });
+
+  describe("buchwert", () => {
+    it("should calculate book value correctly", () => {
+      // 2024: 3333.33 depreciation -> 12000 - 3333.33 = 8666.67
+      expect(buchwert(sampleAsset, 2024)).toBe(8666.67);
+      // 2025: another 4000 -> 8666.67 - 4000 = 4666.67
+      expect(buchwert(sampleAsset, 2025)).toBe(4666.67);
+      // 2026: another 4000 -> 4666.67 - 4000 = 666.67
+      expect(buchwert(sampleAsset, 2026)).toBe(666.67);
+    });
+
+    it("should not go below residual value", () => {
+      const assetWithRestwert: AnlagegutRaw = {
+        anschaffungskosten: 5000,
+        nutzungsdauerJahre: 5,
+        restwert: 500,
+        anschaffungsdatum: "2024-01-01",
+        aktiv: true,
+      };
+      // After full depreciation: 5000 - (900 * 5) = 500
+      expect(buchwert(assetWithRestwert, 2028)).toBe(500);
+      expect(buchwert(assetWithRestwert, 2030)).toBe(500);
+    });
+
+    it("should return acquisition cost for year before acquisition", () => {
+      expect(buchwert(sampleAsset, 2023)).toBe(12000);
+    });
+  });
+
+  describe("assetStatus", () => {
+    it("should return 'inaktiv' for inactive assets", () => {
+      const inactive: AnlagegutRaw = { ...sampleAsset, aktiv: false };
+      expect(assetStatus(inactive, 2025)).toBe("inaktiv");
+    });
+
+    it("should return 'aktiv' for active assets within depreciation period", () => {
+      expect(assetStatus(sampleAsset, 2024)).toBe("aktiv");
+      expect(assetStatus(sampleAsset, 2025)).toBe("aktiv");
+      expect(assetStatus(sampleAsset, 2026)).toBe("aktiv");
+    });
+
+    it("should return 'vollständig abgeschrieben' after depreciation period", () => {
+      expect(assetStatus(sampleAsset, 2027)).toBe("vollständig abgeschrieben");
+      expect(assetStatus(sampleAsset, 2030)).toBe("vollständig abgeschrieben");
+    });
+  });
+});

tests/lib/buchungen.test.ts

@@ -0,0 +1,152 @@
+import { describe, it, expect } from "vitest";
+
+describe("Buchungen - Double-Entry Bookkeeping Logic", () => {
+  describe("TransactionAccount Enum", () => {
+    it("should have KASSE and BANK accounts", () => {
+      // These are the two main accounts for German bookkeeping
+      const accounts = ["KASSE", "BANK"];
+      expect(accounts).toContain("KASSE");
+      expect(accounts).toContain("BANK");
+    });
+  });
+
+  describe("TransactionType Enum", () => {
+    it("should have EINLAGE and ENTNAHME types", () => {
+      // EINLAGE = Owner investment, ENTNAHME = Owner withdrawal
+      const types = ["EINLAGE", "ENTNAHME"];
+      expect(types).toContain("EINLAGE");
+      expect(types).toContain("ENTNAHME");
+    });
+  });
+
+  describe("Zahlungsart Enum", () => {
+    it("should have KASSE and BANK payment methods", () => {
+      const paymentMethods = ["KASSE", "BANK"];
+      expect(paymentMethods).toContain("KASSE");
+      expect(paymentMethods).toContain("BANK");
+    });
+  });
+
+  describe("Buchung Business Logic", () => {
+    it("should calculate correct sign for EINLAGE (positive)", () => {
+      // EINLAGE increases the company's assets
+      const amount = 1000;
+      const type: "EINLAGE" | "ENTNAHME" = "EINLAGE";
+      
+      // In German bookkeeping: EINLAGE is recorded as positive (credit to equity)
+      const signedAmount = type === "EINLAGE" ? amount : -amount;
+      expect(signedAmount).toBe(1000);
+    });
+
+    it("should calculate correct sign for ENTNAHME (negative)", () => {
+      // ENTNAHME decreases the company's assets
+      const amount = 500;
+      const type: "EINLAGE" | "ENTNAHME" = "ENTNAHME";
+      
+      // In German bookkeeping: ENTNAHME is recorded as negative (debit to equity)
+      const signedAmount = type === "EINLAGE" ? amount : -amount;
+      expect(signedAmount).toBe(-500);
+    });
+
+    it("should identify business records correctly", () => {
+      // Business records (isBusinessRecord = true) come from Einnahmen/Ausgaben
+      const isBusinessRecord = true;
+      const hasKategorie = true;
+      
+      expect(isBusinessRecord).toBe(true);
+      expect(hasKategorie).toBe(true);
+    });
+
+    it("should handle non-business records (private)", () => {
+      // Non-business records might not have a kategorie
+      const isBusinessRecord = false;
+      const kategorie = null;
+      
+      expect(isBusinessRecord).toBe(false);
+      expect(kategorie).toBeNull();
+    });
+
+    it("should validate Decimal precision for amounts", () => {
+      // Prisma Decimal(10,2) - max 10 digits, 2 decimal places
+      const amount = 12345678.90; // 8 digits before decimal, 2 after
+      const maxAmount = 99999999.99; // Max for DECIMAL(10,2)
+      
+      expect(amount).toBeLessThanOrEqual(maxAmount);
+      expect(Number(amount.toFixed(2))).toBe(12345678.9);
+    });
+  });
+
+  describe("Buchung Link Logic (Linked Transactions)", () => {
+    it("should allow linking related transactions", () => {
+      // Example: An invoice payment might be linked to the invoice
+      const buchungId = "buchung-123";
+      const linkedBuchungId = "buchung-456";
+      
+      // A Buchung can be linked to another (e.g., invoice payment -> invoice)
+      const link = { source: buchungId, target: linkedBuchungId };
+      expect(link.source).toBe("buchung-123");
+      expect(link.target).toBe("buchung-456");
+    });
+  });
+
+  describe("Kategorie Logic", () => {
+    it("should have unique category names per company", () => {
+      // BuchungKategorie has @@unique([companyId, name, typ])
+      const companyId = "company-123";
+      const categories = [
+        { companyId, name: "Fußpflege", typ: "EINNAHME" },
+        { companyId, name: "Miete", typ: "AUSGABE" },
+      ];
+      
+      const uniqueCheck = new Set(categories.map(c => `${c.companyId}-${c.name}-${c.typ}`));
+      expect(uniqueCheck.size).toBe(categories.length);
+    });
+
+    it("should distinguish between EINNAHME and AUSGABE", () => {
+      const einnahme: "EINNAHME" = "EINNAHME";
+      const ausgabe: "AUSGABE" = "AUSGABE";
+      
+      expect(einnahme).toBe("EINNAHME");
+      expect(ausgabe).toBe("AUSGABE");
+      expect(einnahme).not.toBe(ausgabe);
+    });
+  });
+
+  describe("Date-Based Queries", () => {
+    it("should filter Buchungen by date range", () => {
+      const buchungen = [
+        { date: new Date("2026-01-15"), amount: 100 },
+        { date: new Date("2026-02-20"), amount: 200 },
+        { date: new Date("2026-03-10"), amount: 300 },
+      ];
+      
+      const startDate = new Date("2026-02-01");
+      const endDate = new Date("2026-03-31");
+      
+      const filtered = buchungen.filter(b => 
+        b.date >= startDate && b.date <= endDate
+      );
+      
+      expect(filtered).toHaveLength(2);
+      expect(filtered[0].amount).toBe(200);
+      expect(filtered[1].amount).toBe(300);
+    });
+
+    it("should group Buchungen by month for reports", () => {
+      const buchungen = [
+        { date: new Date("2026-01-10"), amount: 100 },
+        { date: new Date("2026-01-20"), amount: 150 },
+        { date: new Date("2026-02-05"), amount: 200 },
+      ];
+      
+      const grouped = buchungen.reduce((acc, b) => {
+        const month = b.date.getMonth(); // 0-indexed
+        acc[month] = (acc[month] || 0) + b.amount;
+        return acc;
+      }, {} as Record<number, number>);
+      
+      expect(grouped[0]).toBe(250); // January (month 0)
+      expect(grouped[1]).toBe(200); // February (month 1)
+    });
+  });
+});

tests/lib/client-validation.test.ts

@@ -0,0 +1,298 @@
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import {
+  isDebugMode,
+  setDebugMode,
+  validateTaxId,
+  validateVatId,
+  validateIban,
+  validateBic,
+  validateWebsite,
+  validateCompanyForm,
+  getFieldError,
+  hasFieldError,
+  type ValidationError,
+  type CompanyFormData,
+} from "@/lib/client-validation";
+
+describe("client-validation.ts", () => {
+  // Mock localStorage
+  const localStorageMock = (() => {
+    let store: Record<string, string> = {};
+    return {
+      getItem: (key: string) => store[key] || null,
+      setItem: (key: string, value: string) => { store[key] = value; },
+      removeItem: (key: string) => { delete store[key]; },
+      clear: () => { store = {}; },
+    };
+  })();
+
+  beforeEach(() => {
+    // Reset localStorage mock
+    localStorageMock.clear();
+    (global as any).localStorage = localStorageMock;
+    (global as any).window = { localStorage: localStorageMock };
+  });
+
+  afterEach(() => {
+    (global as any).window = undefined;
+  });
+
+  describe("Debug Mode", () => {
+    it("should be disabled by default (no window)", () => {
+      (global as any).window = undefined;
+      expect(isDebugMode()).toBe(false);
+    });
+
+    it("should detect debug mode from localStorage", () => {
+      setDebugMode(true);
+      expect(isDebugMode()).toBe(true);
+    });
+
+    it("should disable debug mode", () => {
+      setDebugMode(true);
+      expect(isDebugMode()).toBe(true);
+      
+      setDebugMode(false);
+      expect(isDebugMode()).toBe(false);
+    });
+  });
+
+  describe("validateTaxId", () => {
+    it("should return null for empty/null values", () => {
+      expect(validateTaxId("")).toBeNull();
+      expect(validateTaxId(null)).toBeNull();
+      expect(validateTaxId(undefined)).toBeNull();
+    });
+
+    it("should validate correct 10-digit tax ID", () => {
+      expect(validateTaxId("1234567890")).toBeNull();
+    });
+
+    it("should reject invalid tax IDs", () => {
+      const result = validateTaxId("123"); // Too short
+      expect(result).not.toBeNull();
+      expect(result?.field).toBe("taxId");
+      expect(result?.message).toContain("10 Ziffern");
+    });
+
+    it("should reject tax ID with letters", () => {
+      const result = validateTaxId("abc4567890");
+      expect(result).not.toBeNull();
+    });
+  });
+
+  describe("validateVatId", () => {
+    it("should return null for empty/null values", () => {
+      expect(validateVatId("")).toBeNull();
+      expect(validateVatId(null)).toBeNull();
+    });
+
+    it("should validate correct DE VAT ID", () => {
+      expect(validateVatId("DE123456789")).toBeNull();
+    });
+
+    it("should reject VAT ID without DE prefix", () => {
+      const result = validateVatId("1234567890");
+      expect(result).not.toBeNull();
+      expect(result?.field).toBe("vatId");
+      expect(result?.message).toContain("DE + 9 Ziffern");
+    });
+
+    it("should reject VAT ID with wrong length", () => {
+      const result = validateVatId("DE12345"); // Too short
+      expect(result).not.toBeNull();
+    });
+  });
+
+  describe("validateIban", () => {
+    it("should return null for empty/null values", () => {
+      expect(validateIban("")).toBeNull();
+      expect(validateIban(null)).toBeNull();
+    });
+
+    it("should validate correct IBAN", () => {
+      expect(validateIban("DE89370400440532013000")).toBeNull();
+    });
+
+    it("should reject invalid IBAN", () => {
+      const result = validateIban("INVALID");
+      expect(result).not.toBeNull();
+      expect(result?.field).toBe("bankIban");
+      expect(result?.message).toContain("IBAN");
+    });
+  });
+
+  describe("validateBic", () => {
+    it("should return null for empty/null values", () => {
+      expect(validateBic("")).toBeNull();
+      expect(validateBic(null)).toBeNull();
+    });
+
+    it("should validate correct BIC", () => {
+      expect(validateBic("DEUTDEFF")).toBeNull();
+    });
+
+    it("should reject invalid BIC", () => {
+      const result = validateBic("123"); // Too short
+      expect(result).not.toBeNull();
+      expect(result?.field).toBe("bankBic");
+    });
+  });
+
+  describe("validateWebsite", () => {
+    it("should return null for empty/null values", () => {
+      expect(validateWebsite("")).toBeNull();
+      expect(validateWebsite(null)).toBeNull();
+    });
+
+    it("should validate correct website URLs", () => {
+      expect(validateWebsite("https://example.com")).toBeNull();
+      expect(validateWebsite("http://example.com")).toBeNull();
+    });
+
+    it("should reject website without protocol", () => {
+      const result = validateWebsite("example.com");
+      expect(result).not.toBeNull();
+      expect(result?.field).toBe("website");
+      expect(result?.message).toContain("http:// oder https://");
+    });
+
+    it("should reject too long website URLs", () => {
+      const longUrl = "https://" + "a".repeat(250);
+      const result = validateWebsite(longUrl);
+      expect(result).not.toBeNull();
+      expect(result?.message).toContain("255");
+    });
+  });
+
+  describe("validateCompanyForm", () => {
+    const validFormData: CompanyFormData = {
+      name: "Test GmbH",
+      address: "Hauptstraße 1",
+      zip: "12345",
+      city: "Berlin",
+    };
+
+    it("should pass validation for valid data", () => {
+      const errors = validateCompanyForm(validFormData);
+      expect(errors).toHaveLength(0);
+    });
+
+    it("should require name", () => {
+      const data = { ...validFormData, name: "" };
+      const errors = validateCompanyForm(data);
+      expect(errors.length).toBeGreaterThan(0);
+      expect(errors[0].field).toBe("name");
+    });
+
+    it("should require address", () => {
+      const data = { ...validFormData, address: "" };
+      const errors = validateCompanyForm(data);
+      const addressError = errors.find(e => e.field === "address");
+      expect(addressError).toBeDefined();
+    });
+
+    it("should require zip", () => {
+      const data = { ...validFormData, zip: "" };
+      const errors = validateCompanyForm(data);
+      const zipError = errors.find(e => e.field === "zip");
+      expect(zipError).toBeDefined();
+    });
+
+    it("should require city", () => {
+      const data = { ...validFormData, city: "" };
+      const errors = validateCompanyForm(data);
+      const cityError = errors.find(e => e.field === "city");
+      expect(cityError).toBeDefined();
+    });
+
+    it("should validate zip format (digits only)", () => {
+      const data = { ...validFormData, zip: "abc" };
+      const errors = validateCompanyForm(data);
+      const zipError = errors.find(e => e.field === "zip");
+      expect(zipError).toBeDefined();
+      expect(zipError?.message).toContain("Zahlen");
+    });
+
+    it("should validate optional fields when provided", () => {
+      const data = {
+        ...validFormData,
+        taxId: "123", // Invalid
+        vatId: "INVALID", // Invalid
+        website: "example.com", // Invalid (no protocol)
+      };
+      const errors = validateCompanyForm(data);
+      expect(errors.length).toBeGreaterThanOrEqual(3);
+    });
+
+    it("should accept valid optional fields", () => {
+      const data = {
+        ...validFormData,
+        taxId: "1234567890",
+        vatId: "DE123456789",
+        website: "https://example.com",
+      };
+      const errors = validateCompanyForm(data);
+      const hasTaxIdError = errors.some(e => e.field === "taxId");
+      const hasVatIdError = errors.some(e => e.field === "vatId");
+      const hasWebsiteError = errors.some(e => e.field === "website");
+      expect(hasTaxIdError).toBe(false);
+      expect(hasVatIdError).toBe(false);
+      expect(hasWebsiteError).toBe(false);
+    });
+
+    it("should validate email format", () => {
+      const data = { ...validFormData, email: "invalid-email" };
+      const errors = validateCompanyForm(data);
+      const emailError = errors.find(e => e.field === "email");
+      expect(emailError).toBeDefined();
+    });
+
+    it("should accept valid email", () => {
+      const data = { ...validFormData, email: "test@example.com" };
+      const errors = validateCompanyForm(data);
+      const emailError = errors.find(e => e.field === "email");
+      expect(emailError).toBeUndefined();
+    });
+
+    it("should validate field length limits", () => {
+      const data = {
+        ...validFormData,
+        name: "a".repeat(300), // Too long
+        phone: "a".repeat(30), // Too long
+      };
+      const errors = validateCompanyForm(data);
+      expect(errors.length).toBeGreaterThan(0);
+    });
+  });
+
+  describe("getFieldError", () => {
+    it("should return error message for field", () => {
+      const errors: ValidationError[] = [
+        { field: "name", message: "Name required" },
+        { field: "email", message: "Invalid email" },
+      ];
+      expect(getFieldError("name", errors)).toBe("Name required");
+      expect(getFieldError("email", errors)).toBe("Invalid email");
+    });
+
+    it("should return null for field without error", () => {
+      const errors: ValidationError[] = [];
+      expect(getFieldError("name", errors)).toBeNull();
+    });
+  });
+
+  describe("hasFieldError", () => {
+    it("should return true if field has error", () => {
+      const errors: ValidationError[] = [
+        { field: "name", message: "Name required" },
+      ];
+      expect(hasFieldError("name", errors)).toBe(true);
+      expect(hasFieldError("email", errors)).toBe(false);
+    });
+
+    it("should return false for empty errors", () => {
+      expect(hasFieldError("name", [])).toBe(false);
+    });
+  });
+});

tests/lib/einnahmen-ausgaben.test.ts

@@ -0,0 +1,116 @@
+import { describe, it, expect } from "vitest";
+import {
+  EINNAHME_KATEGORIEN,
+  EINNAHME_LABELS,
+  type EinnahmeKategorieKey,
+} from "@/lib/einnahmen";
+import {
+  AUSGABE_KATEGORIEN,
+  KATEGORIE_LABELS,
+  type AusgabeKategorieKey,
+} from "@/lib/ausgaben";
+
+describe("einnahmen.ts - Revenue Categories", () => {
+  it("should have all expected categories", () => {
+    expect(EINNAHME_KATEGORIEN).toContain("FUSSPFLEGE");
+    expect(EINNAHME_KATEGORIEN).toContain("PRIVATEINLAGEN");
+    expect(EINNAHME_KATEGORIEN).toContain("DARLEHEN");
+    expect(EINNAHME_KATEGORIEN).toContain("STEUERERSTATTUNGEN");
+    expect(EINNAHME_KATEGORIEN).toContain("ZINSERTRAEGE");
+    expect(EINNAHME_KATEGORIEN).toContain("VERMIETUNG_VERPACHTUNG");
+    expect(EINNAHME_KATEGORIEN).toContain("VERAEUSSERUNGSERLOES");
+    expect(EINNAHME_KATEGORIEN).toContain("EIGENVERBRAUCH");
+    expect(EINNAHME_KATEGORIEN).toContain("SONSTIGE_EINNAHMEN");
+  });
+
+  it("should have 10 revenue categories", () => {
+    expect(EINNAHME_KATEGORIEN).toHaveLength(10);
+  });
+
+  it("should have labels for all categories", () => {
+    EINNAHME_KATEGORIEN.forEach((key) => {
+      expect(EINNAHME_LABELS[key as EinnahmeKategorieKey]).toBeDefined();
+      expect(typeof EINNAHME_LABELS[key as EinnahmeKategorieKey]).toBe("string");
+    });
+  });
+
+  it("should have correct labels", () => {
+    expect(EINNAHME_LABELS.FUSSPFLEGE).toBe("Fußpflege/Verkauf/Gutscheine");
+    expect(EINNAHME_LABELS.PRIVATEINLAGEN).toBe("Privateinlagen");
+    expect(EINNAHME_LABELS.DARLEHEN).toBe("Darlehen");
+    expect(EINNAHME_LABELS.ZINSERTRAEGE).toBe("Zinserträge");
+  });
+
+  it("should have valid TypeScript types", () => {
+    const testKey: EinnahmeKategorieKey = "FUSSPFLEGE";
+    expect(testKey).toBe("FUSSPFLEGE");
+  });
+});
+
+describe("ausgaben.ts - Expense Categories", () => {
+  it("should have all expected categories", () => {
+    expect(AUSGABE_KATEGORIEN).toContain("WAREN_ROHSTOFFE");
+    expect(AUSGABE_KATEGORIEN).toContain("GERINGWERTIGE_WIRTSCHAFTSGUETER");
+    expect(AUSGABE_KATEGORIEN).toContain("ABSCHREIBUNGEN");
+    expect(AUSGABE_KATEGORIEN).toContain("MIETE");
+    expect(AUSGABE_KATEGORIEN).toContain("STROM_WASSER");
+    expect(AUSGABE_KATEGORIEN).toContain("TELEKOMMUNIKATION");
+    expect(AUSGABE_KATEGORIEN).toContain("FORTBILDUNG_MESSEN");
+    expect(AUSGABE_KATEGORIEN).toContain("BEITRAEGE");
+    expect(AUSGABE_KATEGORIEN).toContain("VERSICHERUNGEN");
+    expect(AUSGABE_KATEGORIEN).toContain("WERBEKOSTEN");
+    expect(AUSGABE_KATEGORIEN).toContain("ZINSEN");
+    expect(AUSGABE_KATEGORIEN).toContain("REISEKOSTEN");
+    expect(AUSGABE_KATEGORIEN).toContain("REPARATUREN_INSTANDHALTUNG");
+    expect(AUSGABE_KATEGORIEN).toContain("BUEROBEDARF");
+    expect(AUSGABE_KATEGORIEN).toContain("REPRAESENTATIONSKOSTEN");
+    expect(AUSGABE_KATEGORIEN).toContain("SONSTIGER_BETRIEBSBEDARF");
+    expect(AUSGABE_KATEGORIEN).toContain("NEBENKOSTEN_GELDVERKEHR");
+  });
+
+  it("should have 17 expense categories", () => {
+    expect(AUSGABE_KATEGORIEN).toHaveLength(17);
+  });
+
+  it("should have labels for all categories", () => {
+    AUSGABE_KATEGORIEN.forEach((key) => {
+      expect(KATEGORIE_LABELS[key as AusgabeKategorieKey]).toBeDefined();
+      expect(typeof KATEGORIE_LABELS[key as AusgabeKategorieKey]).toBe("string");
+    });
+  });
+
+  it("should have correct labels", () => {
+    expect(KATEGORIE_LABELS.WAREN_ROHSTOFFE).toBe("Waren, Rohstoffe, Hilfsstoffe");
+    expect(KATEGORIE_LABELS.GERINGWERTIGE_WIRTSCHAFTSGUETER).toBe(
+      "Geringwertige Wirtschaftsgüter"
+    );
+    expect(KATEGORIE_LABELS.MIETE).toBe("Miete");
+    expect(KATEGORIE_LABELS.ZINSEN).toBe("Zinsen");
+  });
+
+  it("should have valid TypeScript types", () => {
+    const testKey: AusgabeKategorieKey = "MIETE";
+    expect(testKey).toBe("MIETE");
+  });
+});
+
+describe("Category Integration", () => {
+  it("should not have overlapping keys between revenue and expenses", () => {
+    const overlap = EINNAHME_KATEGORIEN.filter((key) =>
+      AUSGABE_KATEGORIEN.includes(key as any)
+    );
+    expect(overlap).toHaveLength(0);
+  });
+
+  it("should have consistent naming convention (uppercase with underscores)", () => {
+    const validPattern = /^[A-Z][A-Z0-9]*(_[A-Z0-9]+)*$/;
+    
+    EINNAHME_KATEGORIEN.forEach((key) => {
+      expect(key).toMatch(validPattern);
+    });
+    
+    AUSGABE_KATEGORIEN.forEach((key) => {
+      expect(key).toMatch(validPattern);
+    });
+  });
+});

tests/lib/invoice-number.test.ts

@@ -0,0 +1,63 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import prisma from "@/lib/prisma.server";
+import { generateInvoiceNumber } from "@/lib/invoice-number.server";
+
+// Mock the Prisma client
+vi.mock("@/lib/prisma.server", () => ({
+  default: {
+    company: {
+      update: vi.fn(),
+    },
+  },
+}));
+
+describe("invoice-number.server.ts", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("should generate invoice number with year and sequence", async () => {
+    const mockCompany = {
+      invoicePrefix: "RE",
+      invoiceSequence: 5, // Already incremented by Prisma
+    };
+
+    (prisma.company.update as any).mockResolvedValue(mockCompany);
+
+    const result = await generateInvoiceNumber("company-123");
+
+    // invoiceSequence is already 5 (after increment), so we expect 005
+    expect(result).toBe("RE-2026-005");
+    expect(prisma.company.update).toHaveBeenCalledWith({
+      where: { id: "company-123" },
+      data: { invoiceSequence: { increment: 1 } },
+      select: { invoicePrefix: true, invoiceSequence: true },
+    });
+  });
+
+  it("should pad sequence with zeros", async () => {
+    const mockCompany = {
+      invoicePrefix: "RG",
+      invoiceSequence: 2, // Already incremented by Prisma
+    };
+
+    (prisma.company.update as any).mockResolvedValue(mockCompany);
+
+    const result = await generateInvoiceNumber("company-456");
+
+    expect(result).toBe("RG-2026-002");
+  });
+
+  it("should handle custom prefix", async () => {
+    const mockCompany = {
+      invoicePrefix: "INV",
+      invoiceSequence: 10, // Already incremented by Prisma
+    };
+
+    (prisma.company.update as any).mockResolvedValue(mockCompany);
+
+    const result = await generateInvoiceNumber("company-789");
+
+    expect(result).toBe("INV-2026-010");
+  });
+});

tests/lib/kategorie-defaults.test.ts

@@ -0,0 +1,165 @@
+import { describe, it, expect } from "vitest";
+import {
+  DEFAULT_AUSGABE_KATEGORIEN,
+  DEFAULT_EINNAHME_KATEGORIEN,
+} from "@/lib/kategorie-defaults";
+
+describe("kategorie-defaults.ts", () => {
+  describe("DEFAULT_AUSGABE_KATEGORIEN", () => {
+    it("should have 15 default expense categories", () => {
+      expect(DEFAULT_AUSGABE_KATEGORIEN).toHaveLength(15);
+    });
+
+    it("should include common expense categories", () => {
+      expect(DEFAULT_AUSGABE_KATEGORIEN).toContain("Waren, Rohstoffe, Hilfsstoffe");
+      expect(DEFAULT_AUSGABE_KATEGORIEN).toContain("Miete");
+      expect(DEFAULT_AUSGABE_KATEGORIEN).toContain("Strom, Wasser");
+      expect(DEFAULT_AUSGABE_KATEGORIEN).toContain("Telekommunikationskosten");
+      expect(DEFAULT_AUSGABE_KATEGORIEN).toContain("Bürobedarf");
+    });
+
+    it("should include GERINGWERTIGE_WIRTSCHAFTSGÜTER", () => {
+      expect(DEFAULT_AUSGABE_KATEGORIEN).toContain("Geringwertige Wirtschaftsgüter");
+    });
+
+    it("should include Abschreibungen", () => {
+      expect(DEFAULT_AUSGABE_KATEGORIEN).toContain("Abschreibungen");
+    });
+
+    it("should include Zinsen", () => {
+      expect(DEFAULT_AUSGABE_KATEGORIEN).toContain("Zinsen");
+    });
+
+    it("should include Reisekosten", () => {
+      expect(DEFAULT_AUSGABE_KATEGORIEN).toContain("Reisekosten");
+    });
+
+    it("should have unique values", () => {
+      const unique = new Set(DEFAULT_AUSGABE_KATEGORIEN);
+      expect(unique.size).toBe(DEFAULT_AUSGABE_KATEGORIEN.length);
+    });
+
+    it("should not have empty strings", () => {
+      DEFAULT_AUSGABE_KATEGORIEN.forEach((cat) => {
+        expect(cat).not.toBe("");
+        expect(cat.trim().length).toBeGreaterThan(0);
+      });
+    });
+  });
+
+  describe("DEFAULT_EINNAHME_KATEGORIEN", () => {
+    it("should have 9 default revenue categories", () => {
+      expect(DEFAULT_EINNAHME_KATEGORIEN).toHaveLength(9);
+    });
+
+    it("should include Fußpflege category", () => {
+      expect(DEFAULT_EINNAHME_KATEGORIEN).toContain("Fußpflege/Verkauf/Gutscheine");
+    });
+
+    it("should include Privateinlagen", () => {
+      expect(DEFAULT_EINNAHME_KATEGORIEN).toContain("Privateinlagen");
+    });
+
+    it("should include Darlehen", () => {
+      expect(DEFAULT_EINNAHME_KATEGORIEN).toContain("Darlehen");
+    });
+
+    it("should include Steuererstattungen", () => {
+      expect(DEFAULT_EINNAHME_KATEGORIEN).toContain("Steuererstattungen");
+    });
+
+    it("should include Zinserträge", () => {
+      expect(DEFAULT_EINNAHME_KATEGORIEN).toContain("Zinserträge");
+    });
+
+    it("should include Miet-/Pachteinnahmen", () => {
+      expect(DEFAULT_EINNAHME_KATEGORIEN).toContain("Miet-/Pachteinnahmen");
+    });
+
+    it("should include Veräußerungserlöse", () => {
+      expect(DEFAULT_EINNAHME_KATEGORIEN).toContain("Veräußerungserlöse");
+    });
+
+    it("should have unique values", () => {
+      const unique = new Set(DEFAULT_EINNAHME_KATEGORIEN);
+      expect(unique.size).toBe(DEFAULT_EINNAHME_KATEGORIEN.length);
+    });
+
+    it("should not have empty strings", () => {
+      DEFAULT_EINNAHME_KATEGORIEN.forEach((cat) => {
+        expect(cat).not.toBe("");
+        expect(cat.trim().length).toBeGreaterThan(0);
+      });
+    });
+  });
+
+  describe("Category Integration", () => {
+    it("should not have overlapping categories between income and expenses", () => {
+      const overlap = DEFAULT_AUSGABE_KATEGORIEN.filter((cat) =>
+        DEFAULT_EINNAHME_KATEGORIEN.includes(cat)
+      );
+      expect(overlap).toHaveLength(0);
+    });
+
+    it("should have consistent German language", () => {
+      // Check that categories contain German umlauts or common German words
+      const allCategories = [
+        ...DEFAULT_AUSGABE_KATEGORIEN,
+        ...DEFAULT_EINNAHME_KATEGORIEN,
+      ];
+      
+      allCategories.forEach((cat) => {
+        expect(cat.length).toBeGreaterThan(0);
+        // Should not contain numbers at start (heuristic check)
+        expect(/^\d/.test(cat)).toBe(false);
+      });
+    });
+
+    it("should cover common business categories", () => {
+      // Expenses should cover: rent, utilities, telecom, office supplies
+      const expenseStr = DEFAULT_AUSGABE_KATEGORIEN.join(" ");
+      expect(expenseStr).toContain("Miete");
+      expect(expenseStr).toContain("Bürobedarf");
+      
+      // Revenue should cover: services, loans, interest
+      const revenueStr = DEFAULT_EINNAHME_KATEGORIEN.join(" ");
+      expect(revenueStr).toContain("Darlehen");
+      expect(revenueStr).toContain("Zinserträge");
+    });
+  });
+
+  describe("Practical Usage", () => {
+    it("should be usable as default values for new companies", () => {
+      // Simulate creating default categories for a new company
+      const companyId = "new-company-123";
+      
+      const defaultExpenseCategories = DEFAULT_AUSGABE_KATEGORIEN.map((name) => ({
+        companyId,
+        name,
+        typ: "AUSGABE" as const,
+      }));
+      
+      const defaultRevenueCategories = DEFAULT_EINNAHME_KATEGORIEN.map((name) => ({
+        companyId,
+        name,
+        typ: "EINNAHME" as const,
+      }));
+      
+      expect(defaultExpenseCategories).toHaveLength(15);
+      expect(defaultRevenueCategories).toHaveLength(9);
+      expect(defaultExpenseCategories[0].typ).toBe("AUSGABE");
+      expect(defaultRevenueCategories[0].typ).toBe("EINNAHME");
+    });
+
+    it("should allow adding custom expense categories", () => {
+      const customCategories: string[] = [
+        ...DEFAULT_AUSGABE_KATEGORIEN,
+        "Custom Expense Category",
+      ];
+      
+      expect(customCategories.length).toBe(16); // 15 defaults + 1 custom
+      expect(customCategories[15]).toBe("Custom Expense Category");
+    });
+
+  });
+});

tests/lib/schemas.test.ts

@@ -0,0 +1,245 @@
+import { describe, it, expect } from "vitest";
+
+import {
+  currencySchema,
+  taxRateSchema,
+  ibanSchema,
+  taxIdSchema,
+  vatIdSchema,
+  invoiceSchema,
+  companySchema,
+  customerSchema,
+} from "@/lib/schemas";
+
+describe("schemas.ts - Zod Validation", () => {
+  describe("currencySchema", () => {
+    it("should accept valid currency values", () => {
+      expect(currencySchema.parse(0)).toBe(0);
+      expect(currencySchema.parse(100)).toBe(100);
+      expect(currencySchema.parse(99.99)).toBe(99.99);
+      expect(currencySchema.parse(1000.5)).toBe(1000.5);
+    });
+
+    it("should reject negative values", () => {
+      expect(() => currencySchema.parse(-1)).toThrow("Geldbeträge dürfen nicht negativ sein");
+    });
+
+    it("should reject more than 2 decimal places", () => {
+      expect(() => currencySchema.parse(1.999)).toThrow(
+        "Geldbeträge dürfen maximal 2 Dezimalstellen haben"
+      );
+    });
+
+    it("should accept exactly 2 decimal places", () => {
+      expect(currencySchema.parse(1.99)).toBe(1.99);
+    });
+  });
+
+  describe("taxRateSchema", () => {
+    it("should accept valid German tax rates", () => {
+      expect(taxRateSchema.parse(0)).toBe(0);
+      expect(taxRateSchema.parse(7)).toBe(7);
+      expect(taxRateSchema.parse(19)).toBe(19);
+    });
+
+    it("should reject invalid tax rates", () => {
+      expect(() => taxRateSchema.parse(5)).toThrow(
+        "Steuersatz muss 0 (steuerfrei), 7 (ermäßigt) oder 19 (Standard) sein"
+      );
+      expect(() => taxRateSchema.parse(20)).toThrow();
+      expect(() => taxRateSchema.parse(15)).toThrow();
+    });
+
+    it("should reject non-integer values", () => {
+      expect(() => taxRateSchema.parse(7.5)).toThrow("Steuersatz muss eine ganze Zahl sein");
+    });
+  });
+
+  describe("ibanSchema", () => {
+    it("should accept valid IBANs", () => {
+      expect(ibanSchema.parse("DE89370400440532013000")).toBe("DE89370400440532013000");
+      expect(ibanSchema.parse("AT611904300234573201")).toBe("AT611904300234573201");
+      expect(ibanSchema.parse("CH9300762011623852957")).toBe("CH9300762011623852957");
+    });
+
+    it("should accept empty string", () => {
+      expect(ibanSchema.parse("")).toBe("");
+    });
+
+    it("should reject invalid IBAN format", () => {
+      // No country code - starts with numbers
+      expect(() => ibanSchema.parse("1234567890")).toThrow();
+      // Too short - only 4 chars (need at least 5: 2 letters + 2 digits + 1)
+      expect(() => ibanSchema.parse("DE1")).toThrow();
+    });
+  });
+
+  describe("taxIdSchema", () => {
+    it("should accept valid 10-digit tax IDs", () => {
+      expect(taxIdSchema.parse("1234567890")).toBe("1234567890");
+    });
+
+    it("should accept empty string", () => {
+      expect(taxIdSchema.parse("")).toBe("");
+    });
+
+    it("should reject invalid tax IDs", () => {
+      expect(() => taxIdSchema.parse("123")).toThrow("Steuernummer muss 10 Ziffern haben");
+      expect(() => taxIdSchema.parse("12345678901")).toThrow();
+      expect(() => taxIdSchema.parse("abcdefghij")).toThrow();
+    });
+  });
+
+  describe("vatIdSchema", () => {
+    it("should accept valid DE VAT IDs", () => {
+      expect(vatIdSchema.parse("DE123456789")).toBe("DE123456789");
+    });
+
+    it("should accept empty string", () => {
+      expect(vatIdSchema.parse("")).toBe("");
+    });
+
+    it("should reject invalid VAT IDs", () => {
+      expect(() => vatIdSchema.parse("DE123")).toThrow(
+        "USt-IdNr. muss im Format DE + 9 Ziffern sein"
+      );
+      expect(() => vatIdSchema.parse("1234567890")).toThrow();
+      expect(() => vatIdSchema.parse("DE12345678")).toThrow();
+    });
+  });
+
+  describe("invoiceSchema", () => {
+    const validInvoice = {
+      companyId: "cm123abc",
+      customerId: "cm456def",
+      issueDate: "2026-05-08",
+      dueDate: "2026-06-08",
+      kleinunternehmer: false,
+      items: [
+        {
+          position: 1,
+          description: "Web Development",
+          quantity: 10,
+          unit: "Stunden",
+          unitPrice: 100,
+          taxRate: 19,
+          netAmount: 1000,
+          taxAmount: 190,
+          grossAmount: 1190,
+        },
+      ],
+      netTotal: 1000,
+      taxTotal: 190,
+      grossTotal: 1190,
+    };
+
+    it("should accept valid invoice", () => {
+      const result = invoiceSchema.parse(validInvoice);
+      expect(result.companyId).toBe("cm123abc");
+      expect(result.items).toHaveLength(1);
+    });
+
+    it("should require at least one item", () => {
+      const invalid = { ...validInvoice, items: [] };
+      expect(() => invoiceSchema.parse(invalid)).toThrow(
+        "Mindestens ein Rechnungsposition erforderlich"
+      );
+    });
+
+    it("should accept optional deliveryDate", () => {
+      const withDelivery = { ...validInvoice, deliveryDate: "2026-05-07" };
+      expect(() => invoiceSchema.parse(withDelivery)).not.toThrow();
+    });
+
+    it("should reject invalid dates", () => {
+      const invalid = { ...validInvoice, issueDate: "not-a-date" };
+      expect(() => invoiceSchema.parse(invalid)).toThrow("Ungültiges Datum");
+    });
+
+    it("should accept optional notes with max length", () => {
+      const withNotes = { ...validInvoice, notes: "Test notes" };
+      expect(() => invoiceSchema.parse(withNotes)).not.toThrow();
+    });
+  });
+
+  describe("companySchema", () => {
+    const validCompany = {
+      name: "Test GmbH",
+      taxId: null,
+      vatId: null,
+      address: "Hauptstraße 1",
+      zip: "12345",
+      city: "Berlin",
+      country: "DE",
+      invoicePrefix: "RE",
+      kleinunternehmer: false,
+      bankIban: null,
+      // bankBic is optional, can be omitted or empty string
+    };
+
+    it("should accept valid company", () => {
+      const result = companySchema.parse(validCompany);
+      expect(result.name).toBe("Test GmbH");
+    });
+
+    it("should require name and address", () => {
+      const invalid = { ...validCompany, name: "" };
+      expect(() => companySchema.parse(invalid)).toThrow("Firmenname erforderlich");
+    });
+
+    it("should validate email format", () => {
+      const withEmail = { ...validCompany, email: "test@example.com" };
+      expect(() => companySchema.parse(withEmail)).not.toThrow();
+    });
+
+    it("should reject invalid email", () => {
+      const invalid = { ...validCompany, email: "not-an-email" };
+      expect(() => companySchema.parse(invalid)).toThrow("Ungültige E-Mail-Adresse");
+    });
+
+    it("should validate website starts with http/https", () => {
+      const withWebsite = { ...validCompany, website: "https://example.com" };
+      expect(() => companySchema.parse(withWebsite)).not.toThrow();
+    });
+
+    it("should reject website without protocol", () => {
+      const invalid = { ...validCompany, website: "example.com" };
+      expect(() => companySchema.parse(invalid)).toThrow(
+        "Website muss mit http:// oder https:// beginnen"
+      );
+    });
+
+    it("should accept valid IBAN", () => {
+      const withIban = { ...validCompany, bankIban: "DE89370400440532013000" };
+      expect(() => companySchema.parse(withIban)).not.toThrow();
+    });
+  });
+
+  describe("customerSchema", () => {
+    const validCustomer = {
+      companyId: "cm123abc",
+      name: "Max Mustermann",
+      address: "Musterstraße 1",
+      zip: "12345",
+      city: "Berlin",
+      country: "DE",
+    };
+
+    it("should accept valid customer", () => {
+      const result = customerSchema.parse(validCustomer);
+      expect(result.name).toBe("Max Mustermann");
+    });
+
+    it("should require companyId", () => {
+      const invalid = { ...validCustomer, companyId: "" };
+      expect(() => customerSchema.parse(invalid)).toThrow("Mandant erforderlich");
+    });
+
+    it("should validate zip code format", () => {
+      const invalid = { ...validCustomer, zip: "abc" };
+      expect(() => customerSchema.parse(invalid)).toThrow(
+        "PLZ darf nur Zahlen, Bindestriche und Leerzeichen enthalten"
+      );
+    });
+  });
+});

tests/lib/tax.test.ts

@@ -0,0 +1,160 @@
+import { describe, it, expect } from "vitest";
+
+import {
+  TAX_RATES,
+  calcItemAmounts,
+  calcItemAmountsKleinunternehmer,
+  calcInvoiceTotals,
+  formatCurrency,
+  formatDate,
+} from "@/lib/tax";
+
+describe("tax.ts - German Tax Calculations", () => {
+  describe("TAX_RATES", () => {
+    it("should have correct tax rate values", () => {
+      expect(TAX_RATES).toEqual([
+        { label: "19% MwSt. (Regelsteuersatz)", value: 19 },
+        { label: "7% MwSt. (ermäßigt)", value: 7 },
+        { label: "0% (steuerfrei / §13b UStG)", value: 0 },
+      ]);
+    });
+  });
+
+  describe("calcItemAmounts", () => {
+    it("should calculate correct amounts for 19% tax", () => {
+      const result = calcItemAmounts(2, 100, 19);
+      expect(result).toEqual({
+        netAmount: 200,
+        taxAmount: 38,
+        grossAmount: 238,
+      });
+    });
+
+    it("should calculate correct amounts for 7% tax", () => {
+      const result = calcItemAmounts(1, 50, 7);
+      expect(result).toEqual({
+        netAmount: 50,
+        taxAmount: 3.5,
+        grossAmount: 53.5,
+      });
+    });
+
+    it("should handle tax-free items (0%)", () => {
+      const result = calcItemAmounts(3, 33.33, 0);
+      expect(result).toEqual({
+        netAmount: 99.99,
+        taxAmount: 0,
+        grossAmount: 99.99,
+      });
+    });
+
+    it("should handle decimal quantities", () => {
+      const result = calcItemAmounts(1.5, 100, 19);
+      expect(result).toEqual({
+        netAmount: 150,
+        taxAmount: 28.5,
+        grossAmount: 178.5,
+      });
+    });
+
+    it("should round to 2 decimal places", () => {
+      const result = calcItemAmounts(1, 33.333, 19);
+      // 33.333 * 19% = 6.333..., rounded to 6.33
+      expect(result.netAmount).toBe(33.33);
+      expect(result.taxAmount).toBe(6.33);
+      expect(result.grossAmount).toBe(39.66);
+    });
+  });
+
+  describe("calcItemAmountsKleinunternehmer", () => {
+    it("should set tax to 0 for Kleinunternehmer", () => {
+      const result = calcItemAmountsKleinunternehmer(2, 100);
+      expect(result).toEqual({
+        netAmount: 200,
+        taxAmount: 0,
+        grossAmount: 200,
+      });
+    });
+
+    it("should treat gross as net for Kleinunternehmer", () => {
+      const result = calcItemAmountsKleinunternehmer(1, 50);
+      expect(result.netAmount).toBe(50);
+      expect(result.grossAmount).toBe(50);
+    });
+  });
+
+  describe("calcInvoiceTotals", () => {
+    it("should sum up multiple invoice items", () => {
+      const items = [
+        { netAmount: 100, taxAmount: 19, grossAmount: 119 },
+        { netAmount: 50, taxAmount: 3.5, grossAmount: 53.5 },
+      ];
+      const result = calcInvoiceTotals(items);
+      expect(result).toEqual({
+        netTotal: 150,
+        taxTotal: 22.5,
+        grossTotal: 172.5,
+      });
+    });
+
+    it("should handle empty items array", () => {
+      const result = calcInvoiceTotals([]);
+      expect(result).toEqual({
+        netTotal: 0,
+        taxTotal: 0,
+        grossTotal: 0,
+      });
+    });
+
+    it("should round totals to 2 decimal places", () => {
+      const items = [
+        { netAmount: 33.333, taxAmount: 6.333, grossAmount: 39.666 },
+        { netAmount: 66.666, taxAmount: 12.666, grossAmount: 79.332 },
+      ];
+      const result = calcInvoiceTotals(items);
+      // calcInvoiceTotals uses Math.round which rounds 99.999 to 100
+      expect(result.netTotal).toBe(100);
+      expect(result.taxTotal).toBe(19);
+      expect(result.grossTotal).toBe(119);
+    });
+
+    it("should handle Kleinunternehmer invoice (no tax)", () => {
+      const items = [
+        { netAmount: 100, taxAmount: 0, grossAmount: 100 },
+        { netAmount: 200, taxAmount: 0, grossAmount: 200 },
+      ];
+      const result = calcInvoiceTotals(items);
+      expect(result).toEqual({
+        netTotal: 300,
+        taxTotal: 0,
+        grossTotal: 300,
+      });
+    });
+  });
+
+  describe("formatCurrency", () => {
+    it("should format number to EUR currency", () => {
+      // Note: Intl.NumberFormat uses non-breaking space (\u00A0) before €
+      expect(formatCurrency(1234.56)).toBe("1.234,56\u00A0€");
+    });
+
+    it("should handle string input", () => {
+      expect(formatCurrency("999.99")).toBe("999,99\u00A0€");
+    });
+
+    it("should format zero correctly", () => {
+      expect(formatCurrency(0)).toBe("0,00\u00A0€");
+    });
+  });
+
+  describe("formatDate", () => {
+    it("should format Date object to German format", () => {
+      const date = new Date("2026-05-08");
+      expect(formatDate(date)).toBe("8.5.2026");
+    });
+
+    it("should format date string to German format", () => {
+      expect(formatDate("2026-12-31")).toBe("31.12.2026");
+    });
+  });
+});

vitest.config.ts

@@ -0,0 +1,19 @@
+import { defineConfig } from "vitest/config";
+import react from "@vitejs/plugin-react";
+import tsconfigPaths from "vite-tsconfig-paths";
+
+export default defineConfig({
+  plugins: [react(), tsconfigPaths()],
+  test: {
+    environment: "jsdom",
+    globals: true,
+    setupFiles: ["./vitest.setup.ts"],
+    include: ["tests/**/*.test.ts", "tests/**/*.test.tsx"],
+    coverage: {
+      provider: "v8",
+      reporter: ["text", "json", "html"],
+      include: ["app/lib/**/*.ts", "app/lib/**/*.tsx"],
+      exclude: ["app/lib/**/*.server.ts"],
+    },
+  },
+});

vitest.setup.ts

@@ -0,0 +1,7 @@
+import "@testing-library/jest-dom/vitest";
+
+// Global test setup
+beforeEach(() => {
+  // Clear all mocks before each test
+  vi.clearAllMocks();
+});