Remove manual service worker registration; rely on VitePWA auto-registration

- Deleted src/registerServiceWorker.ts and all related imports
- Cleaned up WebPlatformService and main.web.ts to remove manual SW logic
- Updated VitePWA config for correct dev/prod SW handling
- Fixed missing FontAwesome download icon in PWA prompt
- Updated docs to reflect new PWA registration approach

PWA now works reliably in all web environments with zero manual SW code.

src/services/platforms/WebPlatformService.ts

@@ -647,13 +647,13 @@ export class WebPlatformService implements PlatformService {
 
   // --- PWA/Web-only methods ---
   public registerServiceWorker(): void {
-    if (this.isPWAEnabled) {
-      import("@/registerServiceWorker");
-    }
+    // PWA service worker is automatically registered by VitePWA plugin
+    // No manual registration needed
   }
 
   public get isPWAEnabled(): boolean {
-    return process.env.VITE_PWA_ENABLED === "true";
+    // PWA is always enabled for web platform
+    return true;
   }
 
   /**