audio: reconnect system capture when playback output changes

[conorbronsdon]

Summary

Refs #108.

In All system audio mode, the recorder can follow the current Windows playback output or use a selected fixed output. Switching outputs reconnects the system stream while microphone capture continues. The system track fills reconnection gaps with silence and keeps audio aligned across pause and resume.

The output selector and pause controls stay responsive while a device opens or closes. Test Mic resolves the current default output when Follow Windows default output is selected.

Verification

• Full local test suite: 425 passed.
• Synthetic tests cover output changes, inactive streams, blocked device startup, pause/resume, stop sealing, timeline gaps, and default/fixed/disabled Test Mic choices.
• Independent reviews covered commit a7c60ab: MiniMax M3 through OpenCode Go, plus tool-free Inkling and Ling 3.0 Flash FIN. Findings were checked against source; no verified blocker remains.

Draft pending a hardware check: switch speakers to headphones and back during one recording, include pause/resume and disconnect/reconnect, then listen to the exported audio and check transcript timestamps.