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/PlatformService.ts

@@ -154,4 +154,15 @@ export interface PlatformService {
    * @returns Promise resolving to the first row as an array, or undefined if no results
    */
   dbGetOneRow(sql: string, params?: unknown[]): Promise<unknown[] | undefined>;
+
+  // --- PWA/Web-only methods (optional, only implemented on web) ---
+  /**
+   * Registers the service worker for PWA support (web only)
+   */
+  registerServiceWorker?(): void;
+
+  /**
+   * Returns true if PWA is enabled (web only)
+   */
+  readonly isPWAEnabled?: boolean;
 }