feat: add auto-select best microphone option

[jalvarezz13]

Summary

• Adds an "Auto (best input)" option to the microphone selection dropdown
• Probes all available audio input devices (~300ms each), measures RMS energy, and automatically selects the one with the strongest signal
• Caches the selected device for the session; invalidates on devicechange events
• Falls back to system default if probing fails or no devices are available

Changes

• src/helpers/audioManager.js:

  • New probeDevices() method: opens each mic briefly via getUserMedia() + AnalyserNode, computes peak RMS, returns best deviceId
  • Updated getAudioConstraints(): when selectedMicDeviceId === "auto", runs probe and caches result
  • Constructor: added devicechange listener to clear cache when in auto mode

• src/components/ui/MicrophoneSettings.tsx:

  • Added "Auto (best input)" as first dropdown option before "System Default"
  • Updated SelectValue display for auto mode
  • Added description text when auto is selected

• i18n: Added microphoneSettings.auto and microphoneSettings.autoDescription to all 10 locale files

How it works

1. User selects "Auto (best input)" from the mic dropdown
2. On next recording, getAudioConstraints() detects selectedMicDeviceId === "auto"
3. probeDevices() sequentially opens each audio input for ~300ms, measuring RMS audio energy via AnalyserNode.getByteTimeDomainData()
4. The device with the highest peak RMS wins and is cached
5. Cache is cleared automatically on devicechange events (plug/unplug)

Testing

• npm run build:renderer passes (Vite build clean)
• No new LSP diagnostics
• No settings store changes needed — uses existing selectedMicDeviceId with "auto" as a special value

Closes #514