Linux (PipeWireAudioBridge): Add silence fill during TX. Previously,
feedDaxAudio() returned immediately when transmitting, causing the
named pipe to underrun and creating a timing gap equal to the TX
duration. Now a QTimer feeds elapsed-time-based silence samples to
keep the pipe-source clock advancing in real-time.

macOS (VirtualAudioBridge): Replace fixed 960-sample silence fill with
QElapsedTimer-based sample counting. The old code wrote exactly 20ms
of silence per QTimer tick regardless of actual elapsed time, causing
cumulative drift from timer jitter across TX cycles.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>