runtime.LockOSThread/UnlockOSThread added to withEndpointVolume (volume)
and Capturer.run (viz). COM apartments are thread-affine on Windows;
without the lock the Go scheduler can migrate a goroutine mid-call to a
thread that never called CoInitializeEx, causing sporadic failures.

Also check CoInitializeEx HRESULT: S_OK (0) and S_FALSE (1) are success,
anything else is a real error.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

- internal/volume: IAudioEndpointVolume via COM (pure Go, no CGO)
  - Get/Set master volume (0-100%)
  - GetMute/SetMute
  - Uses WASAPI instead of deprecated MMSystem mixer API
- server: /api/volume now uses system volume (0-100, was Winamp 0-255)
- server: new /api/mute endpoint (GET + POST ?muted=true|false)
- server: volume+mute included in /api/status response
- frontend: mute button with visual state (🔊/🔇, red bar when muted)
- frontend: volume display as percentage label

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>