Turn persistence: onTurnEnd handler creates conversation if needed,
adds user + assistant messages to UI list, caches in Room. Server
already persists via WS-Server — app only needs local state sync.
Model check: CallButton only triggers Live Call when a vertex:*live*
model is in the catalog (liveAvailable). Falls back to PTT otherwise.
Permission: RECORD_AUDIO runtime request via ActivityResultContracts
before starting the call. Denied → toast, no crash.
Title generation: endLiveCall triggers generateTitle + sidebar refresh,
same pattern as text chat.
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