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/WebPlatformService.ts

@@ -645,6 +645,17 @@ export class WebPlatformService implements PlatformService {
     throw new Error("Camera rotation not implemented in web platform");
   }
 
+  // --- PWA/Web-only methods ---
+  public registerServiceWorker(): void {
+    if (this.isPWAEnabled) {
+      import("@/registerServiceWorker");
+    }
+  }
+
+  public get isPWAEnabled(): boolean {
+    return process.env.VITE_PWA_ENABLED === "true";
+  }
+
   /**
    * Checks if running in a worker context
    */