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/apple/macOS/VoiceCatMac/Windows/SettingsWindowController.swift

@@ -63,6 +63,12 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
     private let nrCheckbox = NSButton(checkboxWithTitle: "Noise reduction (RNNoise)",
                                        target: nil, action: nil)
 
+    // Capture the mic in stereo (interleaved L/R) instead of mono. Real stereo only reaches the
+    // wire on a stereo channel; the core folds a stereo mic to mono on a mono channel. Persisted
+    // via MainWindowController.stereoMic.
+    private let stereoMicCheckbox = NSButton(checkboxWithTitle: "Stereo microphone",
+                                             target: nil, action: nil)
+
     // Aux input stream: a second outgoing stream from another hardware input device (e.g. line-in
     // / aux), captured client-side. Device + volume only — aux is always-on. Persisted via
     // MainWindowController.auxEnabled / auxDeviceUID / auxGain.
@@ -178,6 +184,11 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         nrCheckbox.setAccessibilityLabel("Microphone noise reduction")
         nrCheckbox.setAccessibilityHelp("RNNoise denoising of your microphone. Cleans your signal for everyone.")
 
+        stereoMicCheckbox.target = self
+        stereoMicCheckbox.action = #selector(stereoMicChanged)
+        stereoMicCheckbox.setAccessibilityLabel("Stereo microphone")
+        stereoMicCheckbox.setAccessibilityHelp("Capture your microphone in stereo. Only transmitted in stereo on a stereo channel.")
+
         let inputModeRow = NSStackView(views: [inputModeLabel, inputModeControl])
         inputModeRow.orientation = .horizontal
         inputModeRow.spacing = 8
@@ -259,7 +270,7 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         volumeRow.orientation = .horizontal
         volumeRow.spacing = 8
 
-        let stack = NSStackView(views: [inputModeRow, vadRow, inputGainRow, nrCheckbox, pttRow, deviceRow,
+        let stack = NSStackView(views: [inputModeRow, vadRow, inputGainRow, nrCheckbox, stereoMicCheckbox, pttRow, deviceRow,
                                         levelRow, auxHeader, auxCheckbox, auxDeviceRow, auxGainRow,
                                         notificationsHeader, soundsCheckbox, volumeRow,
                                         speechCheckbox, selfTalkCheckbox, pttSoundCheckbox])
@@ -330,6 +341,7 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         inputGainSlider.doubleValue = Double(mc.inputGain * 100)
         updateInputGainLabel()
         nrCheckbox.state = mc.inputNoiseReduction ? .on : .off
+        stereoMicCheckbox.state = mc.stereoMic ? .on : .off
 
         auxCheckbox.state = mc.auxEnabled ? .on : .off
         auxGainSlider.doubleValue = Double(mc.auxGain * 100)
@@ -390,6 +402,16 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         }
     }
 
+    @objc private func stereoMicChanged() {
+        let on = stereoMicCheckbox.state == .on
+        mainController?.stereoMic = on
+        // Channel count only takes effect when the capture device (re)starts, so restart it live.
+        if let mc = mainController, mc.micStreamId != 0 {
+            client.setCaptureChannels(streamId: mc.micStreamId, channels: on ? 2 : 1)
+            client.audioRestart()
+        }
+    }
+
     private func updateInputGainLabel() {
         let pct = Int(inputGainSlider.doubleValue.rounded())
         inputGainValueLabel.stringValue = "\(pct)%"