[Agent] WebRTC: fix ICE gathering/trickle and remove dead signaling code #659

Problem and evidence

web/js/components/preact/WebRTCVideoCell.jsx contains a commented-out trickle-ICE block at ~732-743, which POSTs to /api/webrtc/ice, a route that doesn't exist in LightNVR or go2rtc's HTTP API. Its comment says "If trickle ICE is needed, uncomment the code below". Signaling today is a single HTTP POST {go2rtcBaseUrl}/api/webrtc?src=... with the SDP offer (~814).

Two related cleanups:

• The log says "waiting for ICE gathering...", but it isn't clear whether the code actually waits for iceGatheringState === 'complete' before sending the offer. If it doesn't, the offer may carry no host or srflx candidates. If it does, connecting is slow on networks where gathering stalls (unreachable STUN/TURN gives a timeout of several seconds).
• pc.onicecandidate and nearby code log every candidate with console.log for each cell, which is noisy on a 16-camera grid.

Expected behavior and scope

1. Find out and document what the current flow does: offer sent before or after gathering, and measured time-to-first-frame on LAN.
2. Pick one, and explain why in the PR:
   • (a) Real trickle ICE over go2rtc's WebSocket signaling (/api/ws?src=..., which accepts webrtc/offer and webrtc/candidate messages), proxied through the existing /go2rtc/ path; or
   • (b) Bounded gathering: send the offer when gathering completes or after a short timeout (for example 1 s) with whatever candidates exist.
3. Delete the dead commented block, and move per-candidate logs behind a debug flag (follow existing debug-log conventions if any).

Acceptance criteria

• Time-to-first-frame on LAN is no worse, and is better when STUN is unreachable (simulate with a bogus ICE server from /api/ice-servers).
• WebRTC still connects through the go2rtc proxy path, both with and without authentication.
• No dead commented code is left, and the console is quiet by default.

Validation

• cd web && npx vite build, npm run lint, npm test.
• Manual check in Chrome and Firefox against a running instance. Include before and after timings in the PR.

Constraints and rollback

• Frontend only, unless option (a) needs a WebSocket proxy route the backend doesn't already forward. In that case, say so in the PR and keep the backend change minimal.
• Rollback: revert.