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

package.json

@@ -6,7 +6,11 @@
   "scripts": {
     "dev": "vite",
     "build": "vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest",
+    "test:ui": "vitest --ui",
+    "test:coverage": "vitest --coverage",
+    "test:run": "vitest run"
   },
   "dependencies": {
     "react-dom": "^19.2.4",
@@ -20,6 +24,12 @@
     "@types/node": "^22.14.0",
     "@vitejs/plugin-react": "^5.0.0",
     "typescript": "~5.8.2",
-    "vite": "^6.2.0"
+    "vite": "^6.2.0",
+    "vitest": "^3.0.0",
+    "@vitest/coverage-v8": "^3.0.0",
+    "@testing-library/react": "^16.0.0",
+    "@testing-library/jest-dom": "^6.6.0",
+    "@testing-library/user-event": "^14.5.0",
+    "jsdom": "^26.0.0"
   }
 }