feat: list audio input devices in the menu bar for quick device switching

[lachlanglasgow]

What this changes

Switching the input microphone today requires opening Settings → Audio every time, which is annoying when devices change often (e.g. plugging a laptop into a dock, where the internal mic stops being usable).

This adds a "Show audio devices in menu bar" toggle to Settings → Audio. When enabled, the SpeakType menu bar popover lists all available input microphones, and tapping a device selects it immediately — no need to open Settings.

Details

• Settings → Audio — new standalone "Menu Bar" settings card (below the Input Device card) with the toggle. Persisted via showAudioDevicesInMenuBar, off by default.
• Menu bar popover (MenuBarDashboardView) — new "Input Device" section between the stats grid and the recent transcripts:
  • tappable row per input microphone, with a checkmark and accent border on the active device
  • Refresh button; the list re-fetches every time the popover opens
  • live updates on plug/unplug — AudioRecordingService already observes AVCaptureDevice.wasConnected/wasDisconnected
  • selection persists (same selectedDeviceId UserDefaults path as the settings screen)

Verification

• make build succeeds
• all 73 unit tests pass
• manual: toggling the setting changes the popover height from 492pt → 619pt (section header + 2 device rows on a two-mic machine), and tapping a row switches the active mic instantly

Happy to adjust the wording, the default state, or the section placement if there's a better fit for the project.