feat: integrate PWA functionality with platform service architecture

- Add PWA methods to PlatformService interface (registerServiceWorker, isPWAEnabled)
- Implement PWA logic in WebPlatformService with service worker registration
- Add no-op PWA implementations for Capacitor and Electron platforms
- Create PWAInstallPrompt component with custom install UI and event handling
- Integrate PWA components into App.vue with platform-aware conditional rendering
- Ensure PWA features only load on web platform via platform service pattern
- Centralize PWA logic in platform service for consistent cross-platform behavior
- Add comprehensive PWA documentation and installation flow support

Platform service now handles all PWA operations including service worker
registration, install prompts, and platform-specific feature detection.

src/services/platforms/CapacitorPlatformService.ts

@@ -1299,4 +1299,8 @@ export class CapacitorPlatformService implements PlatformService {
   isWeb(): boolean {
     return false;
   }
+
+  // --- PWA/Web-only methods (no-op for Capacitor) ---
+  public registerServiceWorker(): void {}
+  public get isPWAEnabled(): boolean { return false; }
 }