fix: fall back to legacy sensing websocket

[Safrolomancer]

This keeps the UI working with the legacy Python/macOS sensing server by trying ws://localhost:8765/ws/sensing when same-origin /ws/sensing is unavailable. It also treats macos_wifi as a live source so the UI does not label a real macOS stream as simulated.

[ruvnet]

Closing — superseded by #572 (fix(ui): map sensing websocket port for docker) which landed yesterday. #572 added buildSensingWsUrl() with explicit port-mapping (Docker HTTP 3000 → WS 3001, Python 8080 → WS 8765) and the missing-connection guards in websocket.service.js. The intent of this PR (fallback when the primary WS path is unavailable) is now covered by that port-mapping logic plus the cleanup-race guards. If there's a specific failure mode #572 doesn't cover, please file a focused issue with the reproduction.