The core already supported SCREEN_AUDIO capture on Windows (post-M3 WASAPI
loopback via VOICECAT_HAS_LOOPBACK) and the C# Interop layer was complete
(VcStreamKind.ScreenAudio, StartStream/StopStream/SetRemoteStream). Only the
UI was missing -- no core, proto, or C ABI changes needed.
Adds a 'Share Screen Audio' toggle to the voice panel, independent of mic
voice (can share without joining voice). Disconnect/teardown now stops the
screen stream cleanly. New smoke test exercises the full StartStream ->
StreamStarted -> StopStream -> StreamStopped path through P/Invoke.
Talon
a88656f2fa