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

frontend/vite.config.js

@@ -1,12 +1,31 @@
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
 
+// Parse allowed hosts from environment variable
+const getAllowedHosts = () => {
+  const hosts = process.env.VITE_ALLOWED_HOSTS;
+
+  // If set to 'all', allow all hosts
+  if (hosts === 'all') {
+    return 'all';
+  }
+
+  // If set, parse comma-separated list
+  if (hosts) {
+    return hosts.split(',').map(h => h.trim());
+  }
+
+  // Default: localhost only
+  return ['localhost'];
+};
+
 // https://vite.dev/config/
 export default defineConfig({
   plugins: [react()],
   server: {
     host: '0.0.0.0',
     port: 5173,
+    allowedHosts: getAllowedHosts(),
     watch: {
       usePolling: true,
     },