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.

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"
   }
 }