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/Forms/MainForm.cs

@@ -646,6 +646,7 @@ public partial class MainForm : Form
                 var mode = (VcInputMode)_voiceSettings.InputMode;
                 if (_voiceSettings.InputDeviceId is string devId)
                     _client.SetInputDevice(streamId, devId);
+                _client.SetCaptureChannels(streamId, _voiceSettings.StereoMic ? 2u : 1u);
                 _client.SetInputMode(mode);
                 if (mode == VcInputMode.VoiceActivation)
                     _client.SetVadThreshold(VadThresholdFromSettings());