refactor: implement minimal PWA caching strategy

- Cache only essential PWA assets (manifest, icons, core scripts)
- Use Network First for all other requests
- Remove aggressive caching of UI components and styles
- Preserve PWA installation while minimizing cache footprint

src/scripts/app-boot.js

@@ -3,7 +3,6 @@ import { EnhancedSecureWebRTCManager } from '../network/EnhancedSecureWebRTCMana
 import { EnhancedSecureFileTransfer } from '../transfer/EnhancedSecureFileTransfer.js';
 
 // Import UI components (side-effect: they attach themselves to window.*)
-import '../components/ui/SessionTimer.jsx';
 import '../components/ui/Header.jsx';
 import '../components/ui/DownloadApps.jsx';
 import '../components/ui/UniqueFeatureSlider.jsx';