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/README.md

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+# 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`).