feat(audio): lazy stream close for bluetooth mic latency

[VirenMohindra]

Before Submitting This PR

Please confirm you have done the following:

• I have searched existing issues and pull requests (including closed ones) to ensure this isn't a duplicate
• I have read CONTRIBUTING.md

Human Written Description

• bluetooth microphones (especially airpods) have a 1-2 second activation lag when pressing the transcription shortcut due to CoreAudio device negotiation
• this keeps the mic stream warm for 30 seconds after recording ends in OnDemand mode instead of closing immediately, eliminating that latency for all recordings after the first one
• zero UI / settings changes needed

Related Issues/Discussions

• https://news.ycombinator.com/item?id=46629690

Testing

• record -> stop -> record again within 30s (second should be instant, check logs for "Microphone stream already active")
• record -> stop -> wait ~35s -> record (should see "Closing idle microphone stream" then fresh open)
• multiple quick record / stop cycles (stream stays warm throughout)
• toggle always-on mic on / off while stream is warm (no crashes)
• switch microphone device while stream is warm (stream reopens on new device)
• start recording -> cancel -> verify lazy close scheduled
• test with bluetooth mic if available

AI Assistance

• AI was used (please describe below)

If AI was used:

• Tools used: Claude Code
• How extensively: implementation of the feature based on a pre-planned approach

Screenshots

• successfully builds locally with reduced latency while using airpods on MacOS

[cjpais]

Does this cause audio issues with other applications? That's the main concern I have with this. Cause we're kind of holding a resource that we're not actively using, and I suspect that there could be conflicts with other applications who might try to gain that resource potentially like during video calls or other things. Like I know the audio stuff on Mac OS is pretty weird. And we might just need to ultimately write a better set of drivers for Mac OS to solve this problem at the root level, rather than making patches to the application.