feat: Add comprehensive PWA support with offline functionality

- Add manifest.json with full PWA configuration
  - Support for installation on all platforms (iOS, Android, Desktop)
  - Custom app icons (72x72 to 512x512) with maskable support
  - App shortcuts for quick actions (Create/Join Channel)
  - Protocol handlers for web+securebit:// links
  - Share target integration

- Implement enhanced Service Worker (v4.0)
  - Smart caching strategies (cache-first, network-first, stale-while-revalidate)
  - Security-aware caching (excludes sensitive endpoints)
  - Background sync for failed requests
  - Offline fallbacks with custom error handling
  - Response cloning fixes and CORS handling

- Add PWA Install Prompt Manager
  - Cross-platform install detection and prompts
  - iOS Safari specific installation guide
  - Smart dismissal logic with retry mechanisms
  - Install success notifications and user guidance
  - Persistent install preferences with localStorage

- Implement comprehensive Offline Manager
  - IndexedDB for offline data persistence
  - Automatic message queuing and sync when online
  - Session state recovery after connection loss
  - WebRTC reconnection handling
  - Real-time connection status indicators
  - Offline guidance and help system

- Add offline-first features
  - Message queue with priority and retry logic
  - Session data preservation during disconnection
  - Application state recovery
  - Background sync registration
  - Periodic cleanup of old offline data

- Enhanced user experience
  - Connection status notifications
  - Offline mode guidance and help
  - Automatic sync notifications
  - Reconnection progress indicators
  - Platform-specific installation instructions

This implementation ensures SecureBit.chat works seamlessly offline while maintaining security and providing a native app-like experience across all platforms.

browserconfig.xml

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="utf-8"?>
+<browserconfig>
+    <msapplication>
+        <tile>
+            <square70x70logo src="/logo/ms-icon-70x70.png"/>
+            <square150x150logo src="/logo/ms-icon-150x150.png"/>
+            <square310x310logo src="/logo/ms-icon-310x310.png"/>
+            <wide310x150logo src="/logo/ms-icon-310x150.png"/>
+            <TileColor>#ff6b35</TileColor>
+        </tile>
+        <notification>
+            <polling-uri src="/api/notifications/polling"/>
+            <frequency>30</frequency>
+            <cycle>1</cycle>
+        </notification>
+    </msapplication>
+</browserconfig>