feat(audio): stereo mic capture on Windows & macOS desktop clients

Both desktop mics were hard-mono: the core defaults capture_channels=1 and
neither client ever called vc_set_capture_channels (only iOS did). Add a
persisted "Stereo microphone" toggle to each client's Audio settings, applied
when the mic stream starts and live via vc_set_capture_channels + vc_audio_restart.
Expose both ABI calls in the Windows interop; the macOS wrapper already had them.

Core fix: encode_and_send_frame now folds a stereo mic frame to mono on a mono
channel - previously the channels==2 branch encoded interleaved L/R directly even
on a mono channel, feeding a mono opus_encode 2x its samples (wrong pitch/garbage).
Real stereo still only reaches the wire on a stereo channel; on a mono channel the
mic is cleanly downmixed.

Test: test_stereo_mic_mono_channel. ctest --preset dev green (28/28). Docs: voice.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/windows/VoiceCat.App/Models/VoiceSettings.cs

@@ -26,6 +26,11 @@ public sealed class VoiceSettings
     /// cleaned signal. Independent of the per-listener receive-side NR.</summary>
     public bool MicNoiseReduction { get; set; } = false;
 
+    /// <summary>Capture the mic in stereo (interleaved L/R) instead of mono. Off by default. Real
+    /// stereo only reaches the wire on a stereo channel; on a mono channel the core folds the mic
+    /// to mono. Applied when the capture device next starts (Join Voice or an audio restart).</summary>
+    public bool StereoMic { get; set; } = false;
+
     /// <summary>Push-to-talk key, stored as the integer value of System.Windows.Forms.Keys.</summary>
     public int PttKey { get; set; } = (int)Keys.F8;