feat: Add iOS push notification support (iOS 16.4+)

- Platform detection utilities (is_ios, is_standalone, supports_push)
- iOS-specific meta tags and Apple touch icons
- Auto-detect iOS and show user-friendly message when not in standalone mode
- Enhanced Service Worker with iOS-compatible notification options
- Comprehensive iOS push notification documentation
- manifest.json scope for PWA compliance
- Only works when added to Home Screen (iOS Safari limitation)

frontend/manifest.json

@@ -3,6 +3,7 @@
     "short_name": "VibeTorrent",
     "description": "Modern web-based torrent client with real-time updates",
     "start_url": "/",
+    "scope": "/",
     "display": "standalone",
     "background_color": "#1d232a",
     "theme_color": "#1d232a",