perf: Unbounded audio buffer growth

## What
Implement circular buffering or streaming write for long audio recordings.

## Why
- Audio recording accumulates samples in unbounded Vec buffers
- Long recordings could theoretically exhaust memory
- Located in audio/mod.rs - mic_data and system_data vectors grow indefinitely
- No protection for recordings longer than 2 hours

## How
- Implement circular buffering with configurable max size
- Or stream-to-disk during recording for very long sessions
- Add recording duration warnings
- Consider chunked recording approach