feat: add TURN servers for symmetric NAT traversal

- Add openrelay.metered.ca TURN servers for testing
- Support connections through symmetric NAT (mobile networks)
- Add TCP transport fallback for strict firewalls
- Enables P2P file transfer across different networks

frontend/src/hooks/useWebRTC.js

@@ -1,12 +1,30 @@
 import { useState, useEffect, useRef, useCallback } from 'react';
 import { getSocket } from '../services/socket';
 
-// WebRTC configuration with STUN servers for NAT traversal
+// WebRTC configuration with STUN and TURN servers for NAT traversal
 const rtcConfig = {
   iceServers: [
+    // STUN servers for basic NAT traversal
     { urls: 'stun:stun.l.google.com:19302' },
     { urls: 'stun:stun1.l.google.com:19302' },
     { urls: 'stun:stun2.l.google.com:19302' },
+
+    // TURN servers for symmetric NAT and strict firewalls (public relay for testing)
+    {
+      urls: 'turn:openrelay.metered.ca:80',
+      username: 'openrelayproject',
+      credential: 'openrelayproject',
+    },
+    {
+      urls: 'turn:openrelay.metered.ca:443',
+      username: 'openrelayproject',
+      credential: 'openrelayproject',
+    },
+    {
+      urls: 'turn:openrelay.metered.ca:443?transport=tcp',
+      username: 'openrelayproject',
+      credential: 'openrelayproject',
+    },
   ],
 };