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

tests/setup.ts

@@ -0,0 +1,21 @@
+import '@testing-library/jest-dom';
+import { vi } from 'vitest';
+
+// Mock URL.createObjectURL and URL.revokeObjectURL
+global.URL.createObjectURL = vi.fn(() => 'blob:mock-url');
+global.URL.revokeObjectURL = vi.fn();
+
+// Mock window.matchMedia
+Object.defineProperty(window, 'matchMedia', {
+  writable: true,
+  value: vi.fn().mockImplementation((query: string) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: vi.fn(),
+    removeListener: vi.fn(),
+    addEventListener: vi.fn(),
+    removeEventListener: vi.fn(),
+    dispatchEvent: vi.fn(),
+  })),
+});