fix: prevent sound effects from bleeding into transcription

[jatinkrmalik]

Summary

Fixes sound effects (Pop/Bottle) being picked up by the microphone and transcribed as gibberish words like "Ding", "Pewds", "pulls", "pain", etc.

Root Cause

SoundManager played sounds in parallel with AudioEngine start — the mic captured the system sounds before they finished playing, causing them to be included in the transcription buffer.

Solution

• Made SoundManager.playStartSoundAsync() await sound completion using NSSoundDelegate + CheckedContinuation
• Reordered startRecording() flow: play start sound → await completion → start mic tap
• Stop flow already correct: stop mic tap → play stop sound (fire-and-forget, mic is already off)
• Added 1-second timeout to prevent stuck sounds from blocking recording
• Made SoundManager conform to @unchecked Sendable and added thread-safe locking

Files Changed

• SoundManager.swift — async playback with delegate callback and proper synchronization
• AppState.swift — sequential sound → mic orchestration in startRecording()
• VocaMacTests.swift — tests for async sound behavior (5 new test cases)

Testing

All 41 tests pass, including 5 new SoundManager tests covering:

• Synchronous sound playback (fire-and-forget)
• Asynchronous sound playback with timeout
• Volume control

Technical Details

The fix uses Swift concurrency best practices:

• async/await for cleaner sequencing
• NSSoundDelegate.sound(_:didFinishPlaying:) callback
• CheckedContinuation to bridge delegate to async
• NSLock for thread-safe continuation access
• @unchecked Sendable for SoundManager with manual synchronization

Closes #49