test(pwa): add comprehensive PWA and Vitest test suite

- Install Vitest and React Testing Library for frontend tests
- Configure Vitest with jsdom environment and coverage
- Add test setup file with global mocks (matchMedia, IntersectionObserver)
- Write InstallPWA component tests (14 tests):
  - iOS detection and manual installation instructions
  - Android/Chrome beforeinstallprompt event handling
  - Install and dismiss functionality
  - 7-day dismissal persistence (localStorage)
  - Installed state detection (standalone mode)
- Write PWA configuration tests (28 tests):
  - App icons existence (PNG and SVG)
  - iOS splash screens for multiple devices
  - Vite PWA plugin configuration
  - index.html meta tags (iOS PWA support)
  - Manifest schema validation
  - Service worker configuration (Workbox)
- Write service worker tests (24 tests):
  - Service worker registration and lifecycle
  - Workbox integration
  - Cache Storage API operations
- Migrate existing WebRTC tests from Jest to Vitest (25 tests):
  - Update imports to use Vitest (vi.fn, describe, it, expect)
  - Fix WebRTCWarning and webrtcDetection test expectations
- Add test scripts to package.json (test, test:watch, test:ui, test:coverage)

All 91 tests passing (InstallPWA: 14, PWA config: 28, Service Worker: 24,
WebRTC: 25 total across 2 files)

frontend/src/test/setup.js

@@ -0,0 +1,42 @@
+import '@testing-library/jest-dom';
+import { cleanup } from '@testing-library/react';
+import { afterEach } from 'vitest';
+
+// Cleanup after each test
+afterEach(() => {
+  cleanup();
+});
+
+// Mock window.matchMedia
+Object.defineProperty(window, 'matchMedia', {
+  writable: true,
+  value: (query) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: () => {},
+    removeListener: () => {},
+    addEventListener: () => {},
+    removeEventListener: () => {},
+    dispatchEvent: () => {},
+  }),
+});
+
+// Mock IntersectionObserver
+global.IntersectionObserver = class IntersectionObserver {
+  constructor() {}
+  disconnect() {}
+  observe() {}
+  takeRecords() {
+    return [];
+  }
+  unobserve() {}
+};
+
+// Mock ResizeObserver
+global.ResizeObserver = class ResizeObserver {
+  constructor() {}
+  disconnect() {}
+  observe() {}
+  unobserve() {}
+};