Provides speech-start/speech-end detection via @ricky0123/vad-web.
Retains amplitude-based visual feedback for mic pulse animation but no
longer uses it for stop decisions. Exposes stopAndCheck() which fires
onNoSpeech when the user manually stops without VAD ever detecting
speech — avoids wasted Whisper round-trips on noise-only recordings.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
