feat: Add comprehensive test suite for all services and components

- Set up Vitest with testing-library for React component tests
- Add 20 tests for pdfService (field extraction, PDF filling, visual overlay)
- Add 14 tests for geminiService with mocked API responses
- Add 17 tests for FileUpload component (drag-drop, file selection, preview)
- Add 28 tests for ReviewPanel component (rendering, editing, filtering)
- Add 21 Python tests for fill_pdf.py (extraction, filling, CLI)

Total: 100 tests covering critical functionality

https://claude.ai/code/session_01Wi3BtYKgQu6v4zbydtG6Sy

vitest.config.ts

@@ -0,0 +1,23 @@
+import { defineConfig } from 'vitest/config';
+import react from '@vitejs/plugin-react';
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./tests/setup.ts'],
+    include: ['tests/**/*.test.{ts,tsx}'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'json', 'html'],
+      include: ['services/**/*.ts', 'components/**/*.tsx'],
+      exclude: ['**/node_modules/**', '**/tests/**']
+    }
+  },
+  resolve: {
+    alias: {
+      '@': '/home/user/Rentenversicherer'
+    }
+  }
+});