Start MLX capture off the main thread to avoid hotkey freeze

[yxs]

Problem

With the local MLX engine, pressing the Fn hotkey to dictate could freeze the app and leave Fn unresponsive, with no overlay feedback.

Root cause: the global hotkey's CGEventTap callback is attached to the main run loop (HotkeyManager → CFRunLoopAddSource(CFRunLoopGetMain(), …)), and MLXTranscriber.startRecording() started AVAudioEngine synchronously on that thread. When CoreAudio is wedged — e.g. coreaudiod stuck after a Continuity/iPhone mic hotplug — AVAudioEngine.start() blocks ~10s and then fails with kAudioHardwareNotRunningError ('stop', on PerformCommand(kAUStartIO)). During those 10s the main thread is blocked, so:

• the UI freezes and the recording overlay never paints, and
• the event tap exceeds its timeout and macOS disables it (kCGEventTapDisabledByTimeout), so the Fn hotkey stops working until it is re-armed.

The failure was also only written to the log and never surfaced — unlike the WhisperKit, Speech and Remote paths, which already kick their start onto a Task and report start failures to the overlay. Only the MLX path was synchronous.

Fix

• Move the blocking AVAudioEngine.start() off the main actor and race it against a 6s timeout, so a wedged audio device can no longer freeze the hotkey/UI thread.
• Convert MLX start to the same async startRecordingSession() async -> String? shape the other engines use, surfacing failures via handleRecordingStartFailure (overlay reminder + clean reset) instead of silently resetting.
• Guard against a stray capture if the user releases Fn while the engine is still starting (mirrors shouldContinueWhisperStartup).

Notes for review

• The off-main start uses a withThrowingTaskGroup race (same idiom as RemoteProviderConnectivityTester). On timeout it calls engine.stop() to unwind the blocked start(); that is the one place start/stop can briefly overlap, and it only happens on the wedged-HAL error path. Flag if you'd prefer a different abort mechanism.
• The underlying wedged-coreaudiod condition is environmental (cleared by killall coreaudiod); this PR only makes the app degrade gracefully instead of freezing.

Testing

• xcodebuild -scheme Voxt -configuration Debug → BUILD SUCCEEDED, no new warnings.
• The wedged-HAL state could not be reproduced on demand, so the timeout/abort path is verified by code inspection + build only; the happy path is unchanged.

[yxs]

@hehehai fix with code agent, please review/amend before merge.