feat: WebRTC broadcast with audio

[mohitgauniyal]

Replace JPEG-over-socket frame streaming with WebRTC peer connections for smooth, low-latency broadcast.

Changes:

• WebRTC signalling via existing Socket.IO connection (offer/answer/ICE relay)
• Broadcaster maintains one peer connection per viewer — mesh architecture, no new infrastructure
• Google STUN servers for LAN peer discovery — no TURN needed for same-network use
• Mic captured via getUserMedia and mixed into broadcast stream, muted by default on viewer side
• Viewer mute/unmute toggle added to panel
• Capture to channel works via canvas snapshot of live video frame
• Broadcaster and viewer both clean up correctly on server shutdown or tab close
• All existing broadcast tests pass unchanged — HTTP endpoints untouched