fix(apple-client/audio): capture the right channel of a multi-channel mic + diagnostics

The mic uplink handed the host pure digital silence on a multi-channel
interface: AVAudioConverter's N→stereo downmix takes channels 0/1, but a
pro interface puts the mic on ONE higher discrete channel. Fold the input
to a mono bus ourselves instead — pick the mic's channel (or sum all) and
resample that to the encoder's 48 kHz stereo, so the silent 0/1 downmix
never happens.

- New "Microphone channel" setting (macOS): Auto (sum every channel — a
  lone hot mic passes at full level) or pin 1-based channel N. Picker
  appears only for multi-channel devices, driven by the device's input
  channel count.
- Diagnostics that make this class of failure self-naming next session:
  log the actual live capture device + format + fold mode, warn on a
  silent UID fallback, and a one-shot silence tripwire on the EXTRACTED
  signal (WARN on 10 s of zeros, else peak dBFS).
- foldToMono extracted as a pure, unit-tested helper (pin / sum-clamp x
  interleaved / deinterleaved / mono / out-of-range).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/apple/Sources/PunktfunkClient/Settings/SettingsView.swift

@@ -61,8 +61,12 @@ struct SettingsView: View {
     #if os(macOS)
     @AppStorage(DefaultsKey.speakerUID) var speakerUID = ""
     @AppStorage(DefaultsKey.micUID) var micUID = ""
+    @AppStorage(DefaultsKey.micChannel) var micChannel = 0
     @State var outputDevices: [AudioDevice] = []
     @State var inputDevices: [AudioDevice] = []
+    // Input channels of the selected mic — drives the "Microphone channel" picker, which only
+    // appears for a multi-channel interface (>1). 0 until the Audio tab loads it.
+    @State var micChannelCount = 0
     #endif
 
     #if os(iOS)
@@ -115,6 +119,12 @@ struct SettingsView: View {
             .onAppear {
                 outputDevices = AudioDevices.outputs()
                 inputDevices = AudioDevices.inputs()
+                micChannelCount = AudioDevices.inputChannelCount(forUID: micUID)
+            }
+            .onChange(of: micUID) { _, newUID in
+                // A different mic → different channel count; drop a now-out-of-range pin to Auto.
+                micChannelCount = AudioDevices.inputChannelCount(forUID: newUID)
+                if micChannel > micChannelCount { micChannel = 0 }
             }
             .tabItem { Label("Audio", systemImage: "speaker.wave.2") }