feat(stt): warm audio pipeline on prepare and retain between recordings

[chriswritescode-dev]

Add prepare() method to AudioRecorder to warm the audio context and worklet before first recording. Retain the audio pipeline between recordings (only the mic track is stopped after each use). Full resources are released via dispose() when STT is disabled or the component unmounts.

• Add AudioRecorder.prepare() to pre-warm audio context and worklet
• Keep audio context alive across recordings; only stop mic track between uses
• Add AudioRecorder.dispose() for full cleanup of all resources
• Refactor cleanup() → cleanupRecording(closeAudioContext) to distinguish partial vs full cleanup
• Fix race condition: dispose() during async start() prevents entering recording state
• Update useSTT to use disposeAudioRecorder on unmount and timeout
• Document the performance improvement in docs/features/stt.md
• Test coverage for prepare, reuse across recordings, lifecycle cancellation

Files

• docs/features/stt.md
• frontend/src/hooks/useSTT.ts
• frontend/src/hooks/useSTT.test.tsx
• frontend/src/lib/audioRecorder.ts
• frontend/src/lib/audioRecorder.test.ts