Pause playback while transmitting and declare microphone use.

macOS refuses capture without a usage string, and queued remote audio was still handed to the speaker after push-to-talk began.

Co-authored-by: Cursor <cursoragent@cursor.com>

station/station.go

@@ -151,6 +151,7 @@ func (s *Station) WriteMicrophone(pcm []int16) {
 		if err != nil {
 			continue
 		}
+		// A lost datagram ends the session when the receive timer expires.
 		_ = s.sendUDP(udpAudio, appendBlob(nil, pkt))
 	}
 }
@@ -159,6 +160,10 @@ func (s *Station) WriteMicrophone(pcm []int16) {
 func (s *Station) TakePlayback() []int16 {
 	s.mu.Lock()
 	defer s.mu.Unlock()
+	if s.transmitting() {
+		s.play = nil
+		return nil
+	}
 	out := s.play
 	s.play = nil
 	return out