Fix browser phone connection and audio issues on Android tablets

Resolves issues where browser phone PWA failed to connect and calls would
immediately hang up when answered on Android tablets. Adds proper mobile
audio handling, device connection monitoring, and PWA notifications for
incoming calls.

Key changes:
- Add AudioContext initialization with mobile unlock for autoplay support
- Add Android-specific WebRTC constraints (echo cancellation, ICE restart)
- Add device connection state monitoring and auto-reconnection
- Add incoming call ringtone with vibration fallback
- Add PWA service worker notifications for background calls
- Add Page Visibility API for background call detection
- Improve call answer handler with connection state validation
- Add touch event support for mobile dialpad

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

assets/sounds/README.md

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+# Ringtone Audio Files
+
+## Custom Ringtone
+
+To add a custom ringtone for incoming calls in the browser phone:
+
+1. Place your ringtone audio file in this directory
+2. Name it: `ringtone.mp3`
+3. Recommended format: MP3, under 5 seconds, loopable
+
+## Fallback Behavior
+
+If no ringtone file is present, the browser phone will:
+- Use device vibration on mobile devices (Android, iOS)
+- Rely on browser notifications to alert users
+- Display visual indicators in the browser interface
+
+## Supported Audio Formats
+
+- **MP3** (recommended) - Best compatibility across browsers
+- **OGG** - Good for Firefox, Chrome
+- **WAV** - Larger file size but universal support
+
+## File Requirements
+
+- Maximum file size: 100KB recommended
+- Duration: 1-5 seconds (will loop)
+- Sample rate: 44.1kHz or 48kHz
+- Bitrate: 128kbps or higher
+
+## Example Ringtone Sources
+
+You can find free ringtone files at:
+- https://freesound.org/
+- https://incompetech.com/
+- Or create your own using Audacity or similar tools