fix(audio): eliminate echo loopback and handle websocket disconnect

[PyDevDeep]

Goal

Fix acoustic echo and eliminate websocket termination errors when closing the session.

Changes

• Removed local microphone loopback to audioCtx.destination in useVoiceSession.js to prevent acoustic feedback into the LLM.
• Handled RuntimeError gracefully in ws_voice.py when websocket.receive() gets called after the connection drops abruptly.

How to Test

1. Checkout the branch fix/audio-echo-and-disconnect.
2. Start the FastAPI backend and frontend.
3. Open the voice chat UI and begin speaking.
4. Verify there is no echo or continuous noise feedback.
5. Close the browser tab mid-conversation and check backend logs (there should be no unhandled errors).