feat: implement WebRTC P2P file transfer with DataChannel

Add complete WebRTC peer-to-peer file transfer functionality:

Backend changes:
- Add WebRTC signaling events to Socket.IO (offer, answer, ICE candidates)
- Implement authorization checks for match participants
- Add signaling relay between matched users

Frontend changes:
- Create useWebRTC hook for RTCPeerConnection management
- Implement RTCDataChannel with 16KB chunking for large files
- Add real-time progress monitoring for sender and receiver
- Implement automatic file download on receiver side
- Add connection state tracking and error handling
- Integrate WebRTC with MatchChatPage (replace mockup)

Configuration:
- Add Vite allowed hosts configuration via VITE_ALLOWED_HOSTS env var
- Support comma-separated host list or 'all' for development
- Add .env.example with configuration examples
- Update docker-compose.yml with default allowed hosts

Documentation:
- Add comprehensive WebRTC testing guide with troubleshooting
- Add quick test checklist for manual testing
- Document WebRTC flow, requirements, and success criteria

Features:
- End-to-end encrypted P2P transfer (DTLS)
- 16KB chunk size optimized for DataChannel
- Buffer management to prevent overflow
- Automatic connection establishment with 30s timeout
- Support for files of any size
- Real-time progress tracking
- Clean connection lifecycle management

.env.example

@@ -0,0 +1,7 @@
+# Frontend - Vite Allowed Hosts
+# Comma-separated list of allowed hostnames
+# Use 'all' to allow all hosts (NOT recommended for production)
+VITE_ALLOWED_HOSTS=localhost,spotlight.cam,.spotlight.cam
+
+# Alternative: Allow all hosts (development only)
+# VITE_ALLOWED_HOSTS=all