- _waitForPlayback now only succeeds on playing/timeupdate events, not
loadeddata/canplay which fire from just the WAV header before real
audio arrives
- stopMonitor() pauses audio and updates UI immediately instead of
blocking on the backend stop request (1+ second delay)
- Reduced backend audio stop sleep from 1.0s to 0.15s; the start
retry loop already handles USB contention
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>