feat: switch to STUN servers for production NAT traversal

Changed WebRTC config from localhost-only to STUN servers:
- Removed rtcConfigLocalhost (no longer needed)
- Using rtcConfig with Google STUN servers
- Enables NAT traversal for users on different networks
- Removed unnecessary iceCandidatePoolSize config
- Link sharing remains as fallback for blocked users

frontend/src/hooks/useWebRTC.js

@@ -1,20 +1,13 @@
 import { useState, useEffect, useRef, useCallback } from 'react';
 import { getSocket } from '../services/socket';
 
-// WebRTC configuration with STUN servers
+// WebRTC configuration with STUN servers for NAT traversal
 const rtcConfig = {
   iceServers: [
     { urls: 'stun:stun.l.google.com:19302' },
     { urls: 'stun:stun1.l.google.com:19302' },
     { urls: 'stun:stun2.l.google.com:19302' },
   ],
-  iceTransportPolicy: 'all', // Use all candidates (host, srflx, relay)
-  iceCandidatePoolSize: 10, // Pre-gather candidates
-};
-
-// Alternative config for localhost testing (no STUN)
-const rtcConfigLocalhost = {
-  iceServers: [], // No STUN - use only host candidates for localhost
 };
 
 // File chunk size (16KB recommended for WebRTC DataChannel)
@@ -68,10 +61,10 @@ export const useWebRTC = (matchId, userId) => {
       return peerConnectionRef.current;
     }
 
-    // Use localhost config for testing (no STUN servers)
-    const pc = new RTCPeerConnection(rtcConfigLocalhost);
+    // Use full config with STUN servers for production
+    const pc = new RTCPeerConnection(rtcConfig);
     peerConnectionRef.current = pc;
-    console.log('🔧 Using rtcConfigLocalhost (no STUN servers)');
+    console.log('🔧 Using rtcConfig with STUN servers for NAT traversal');
 
     // ICE candidate handler
     pc.onicecandidate = (event) => {