Add comprehensive tests for client validation, revenue and expense categories, invoice generation, and schemas

- Implement tests for client validation functions including tax ID, VAT ID, IBAN, BIC, website, and company form validation.
- Create tests for revenue and expense categories ensuring all expected categories and labels are present.
- Add tests for invoice number generation with various scenarios including prefix handling and sequence padding.
- Introduce tests for default categories and their integration, ensuring no overlaps and consistent naming conventions.
- Implement Zod schema validation tests for currency, tax rates, IBAN, tax ID, VAT ID, invoices, companies, and customers.
- Add utility tests for tax calculations, including item amounts and invoice totals, ensuring correct handling of tax rates and formatting.
- Set up Vitest configuration and global test setup for consistent testing environment.

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");
+    });
+  });
+});