feat: implement WebRTC P2P file transfer with detection and fallback

Implemented complete WebRTC peer-to-peer file transfer system for match chat:

**Core WebRTC Implementation:**
- Created useWebRTC hook with RTCPeerConnection and RTCDataChannel
- P2P file transfer with 16KB chunking for large files (tested up to 700MB)
- Real-time progress monitoring for sender and receiver
- Automatic file download on receiver side
- End-to-end encryption via DTLS (native WebRTC)
- ICE candidate exchange via Socket.IO signaling
- Support for host candidates (localhost testing)

**WebRTC Detection & User Experience:**
- Automatic WebRTC capability detection on page load
- Detects if ICE candidates can be generated (fails in Opera, privacy-focused browsers, VPNs)
- User-friendly warning component with fix suggestions
- Graceful degradation: disables WebRTC button when blocked
- Suggests alternative methods (video links via Google Drive/Dropbox)

**Socket.IO Improvements:**
- Fixed multiple socket instance creation issue
- Implemented socket instance reuse pattern
- Disabled React.StrictMode to prevent reconnection loops in development

**Technical Details:**
- RTCPeerConnection with configurable STUN servers (currently using localhost config)
- RTCDataChannel with ordered delivery
- Comprehensive logging for debugging (ICE gathering, connection states, signaling)
- Match room-based signaling relay via Socket.IO
- Authorization checks for all WebRTC signaling events

**Files Changed:**
- frontend/src/hooks/useWebRTC.js - Complete WebRTC implementation
- frontend/src/utils/webrtcDetection.js - WebRTC capability detection
- frontend/src/components/WebRTCWarning.jsx - User warning component
- frontend/src/pages/MatchChatPage.jsx - WebRTC integration
- frontend/src/services/socket.js - Socket instance reuse
- frontend/src/main.jsx - Disabled StrictMode for Socket.IO stability

**Testing:**
- ✅ Verified working in Chrome (ICE candidates generated)
- ✅ Tested with 700MB file transfer
- ✅ Detection working in Opera (shows warning when WebRTC blocked)
- ✅ P2P connection establishment and DataChannel opening
- ✅ File chunking and progress monitoring

**TODO:**
- Add STUN server configuration for production (NAT traversal)
- Consider server-based upload fallback for blocked users

frontend/src/hooks/useWebRTC.js

@@ -8,6 +8,13 @@ const rtcConfig = {
     { 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)
@@ -29,6 +36,14 @@ export const useWebRTC = (matchId, userId) => {
   const peerConnectionRef = useRef(null);
   const dataChannelRef = useRef(null);
   const socketRef = useRef(null);
+  const matchIdRef = useRef(matchId);
+  const userIdRef = useRef(userId);
+
+  // Update refs when props change
+  useEffect(() => {
+    matchIdRef.current = matchId;
+    userIdRef.current = userId;
+  }, [matchId, userId]);
 
   // File transfer state
   const fileTransferRef = useRef({
@@ -53,20 +68,39 @@ export const useWebRTC = (matchId, userId) => {
       return peerConnectionRef.current;
     }
 
-    const pc = new RTCPeerConnection(rtcConfig);
+    // Use localhost config for testing (no STUN servers)
+    const pc = new RTCPeerConnection(rtcConfigLocalhost);
     peerConnectionRef.current = pc;
+    console.log('🔧 Using rtcConfigLocalhost (no STUN servers)');
 
     // ICE candidate handler
     pc.onicecandidate = (event) => {
-      if (event.candidate && socketRef.current) {
-        socketRef.current.emit('webrtc_ice_candidate', {
-          matchId,
-          candidate: event.candidate,
-        });
-        console.log('📤 Sent ICE candidate');
+      if (event.candidate) {
+        console.log('🧊 ICE candidate generated:', event.candidate.type);
+        if (socketRef.current) {
+          socketRef.current.emit('webrtc_ice_candidate', {
+            matchId: matchIdRef.current,
+            candidate: event.candidate,
+          });
+          console.log('📤 Sent ICE candidate');
+        } else {
+          console.error('❌ Socket not available to send ICE candidate');
+        }
+      } else {
+        console.log('🧊 ICE gathering complete (candidate is null)');
       }
     };
 
+    // ICE gathering state handler
+    pc.onicegatheringstatechange = () => {
+      console.log('🧊 ICE gathering state:', pc.iceGatheringState);
+    };
+
+    // Signaling state handler
+    pc.onsignalingstatechange = () => {
+      console.log('📡 Signaling state:', pc.signalingState);
+    };
+
     // Connection state change handler
     pc.onconnectionstatechange = () => {
       console.log('🔄 Connection state:', pc.connectionState);
@@ -84,8 +118,9 @@ export const useWebRTC = (matchId, userId) => {
     };
 
     console.log('✅ RTCPeerConnection initialized');
+    console.log('🔍 Initial states - Connection:', pc.connectionState, 'ICE:', pc.iceConnectionState, 'Signaling:', pc.signalingState);
     return pc;
-  }, [matchId]);
+  }, []);
 
   /**
    * Create data channel for file transfer
@@ -200,9 +235,11 @@ export const useWebRTC = (matchId, userId) => {
 
       const offer = await pc.createOffer();
       await pc.setLocalDescription(offer);
+      console.log('✅ Local description set (offer). ICE gathering should start now...');
+      console.log('🔍 SDP has candidates:', pc.localDescription.sdp.includes('candidate:'));
 
       socketRef.current.emit('webrtc_offer', {
-        matchId,
+        matchId: matchIdRef.current,
         offer: pc.localDescription,
       });
 
@@ -211,7 +248,7 @@ export const useWebRTC = (matchId, userId) => {
       console.error('Failed to create offer:', error);
       setConnectionState('failed');
     }
-  }, [matchId, initializePeerConnection, createDataChannel]);
+  }, [initializePeerConnection, createDataChannel]);
 
   /**
    * Handle incoming offer and create answer
@@ -230,12 +267,15 @@ export const useWebRTC = (matchId, userId) => {
       };
 
       await pc.setRemoteDescription(new RTCSessionDescription(offer));
+      console.log('✅ Remote description set (offer)');
 
       const answer = await pc.createAnswer();
       await pc.setLocalDescription(answer);
+      console.log('✅ Local description set (answer). ICE gathering should start now...');
+      console.log('🔍 SDP has candidates:', pc.localDescription.sdp.includes('candidate:'));
 
       socketRef.current.emit('webrtc_answer', {
-        matchId,
+        matchId: matchIdRef.current,
         answer: pc.localDescription,
       });
 
@@ -244,7 +284,7 @@ export const useWebRTC = (matchId, userId) => {
       console.error('Failed to handle offer:', error);
       setConnectionState('failed');
     }
-  }, [matchId, initializePeerConnection, setupDataChannelHandlers]);
+  }, [initializePeerConnection, setupDataChannelHandlers]);
 
   /**
    * Handle incoming answer
@@ -258,7 +298,7 @@ export const useWebRTC = (matchId, userId) => {
       }
 
       await pc.setRemoteDescription(new RTCSessionDescription(answer));
-      console.log('✅ Remote description set');
+      console.log('✅ Remote description set (answer). ICE should connect now...');
     } catch (error) {
       console.error('Failed to handle answer:', error);
       setConnectionState('failed');
@@ -385,36 +425,67 @@ export const useWebRTC = (matchId, userId) => {
 
     socketRef.current = socket;
 
-    // Listen for WebRTC signaling events
-    socket.on('webrtc_offer', ({ from, offer }) => {
-      console.log('📥 Received WebRTC offer from:', from);
-      if (from !== userId) {
+    // Create stable handlers using current values from refs
+    const onOffer = ({ from, offer }) => {
+      console.log('📥 Received WebRTC offer from:', from, 'My userId:', userIdRef.current);
+      if (from !== userIdRef.current) {
         handleOffer(offer);
+      } else {
+        console.log('⏭️ Ignoring offer from self');
       }
-    });
+    };
 
-    socket.on('webrtc_answer', ({ from, answer }) => {
-      console.log('📥 Received WebRTC answer from:', from);
-      if (from !== userId) {
+    const onAnswer = ({ from, answer }) => {
+      console.log('📥 Received WebRTC answer from:', from, 'My userId:', userIdRef.current);
+      if (from !== userIdRef.current) {
         handleAnswer(answer);
+      } else {
+        console.log('⏭️ Ignoring answer from self');
       }
-    });
+    };
 
-    socket.on('webrtc_ice_candidate', ({ from, candidate }) => {
+    const onIceCandidate = ({ from, candidate }) => {
       console.log('📥 Received ICE candidate from:', from);
-      if (from !== userId) {
+      if (from !== userIdRef.current) {
         handleIceCandidate(candidate);
       }
-    });
+    };
 
-    // Cleanup on unmount
+    // Attach listeners
+    const attachListeners = () => {
+      socket.off('webrtc_offer', onOffer);
+      socket.off('webrtc_answer', onAnswer);
+      socket.off('webrtc_ice_candidate', onIceCandidate);
+
+      socket.on('webrtc_offer', onOffer);
+      socket.on('webrtc_answer', onAnswer);
+      socket.on('webrtc_ice_candidate', onIceCandidate);
+
+      console.log('✅ WebRTC Socket.IO listeners attached (socketId:', socket.id, ')');
+    };
+
+    // Attach initially
+    attachListeners();
+
+    // Reattach on reconnect
+    const onReconnect = () => {
+      console.log('🔄 Socket reconnected - reattaching WebRTC listeners');
+      attachListeners();
+    };
+
+    socket.on('connect', onReconnect);
+
+    // Cleanup on unmount only
     return () => {
-      socket.off('webrtc_offer');
-      socket.off('webrtc_answer');
-      socket.off('webrtc_ice_candidate');
+      console.log('🧹 Removing WebRTC Socket.IO listeners');
+      socket.off('connect', onReconnect);
+      socket.off('webrtc_offer', onOffer);
+      socket.off('webrtc_answer', onAnswer);
+      socket.off('webrtc_ice_candidate', onIceCandidate);
       cleanupConnection();
     };
-  }, [userId, handleOffer, handleAnswer, handleIceCandidate, cleanupConnection]);
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [userId]); // Only re-run if userId changes
 
   return {
     connectionState,