feat: microphone device picker

The webcam had a device picker but the mic was hardcoded to the system
default. Settings' Audio section now offers a microphone picker
(persisted like the camera choice) and the capture pipeline resolves the
selected device by uniqueID, falling back to the system default when the
device disappears.

Author: jasonzh0

CineScreen/App/AppState.swift

@@ -24,6 +24,8 @@ final class AppState {
     var captureCamera: Bool = false
     /// AVCaptureDevice uniqueID for the selected webcam. nil = system default.
     var selectedCameraID: String? = nil
+    /// AVCaptureDevice uniqueID for the selected microphone. nil = default.
+    var selectedMicID: String? = nil
 
     // Permissions snapshot (refreshed on a timer and after each request)
     var permissions: PermissionStatus
@@ -60,6 +62,7 @@ final class AppState {
         static let captureMic = "cs.captureMic"
         static let captureCamera = "cs.captureCamera"
         static let selectedCameraID = "cs.selectedCameraID"
+        static let selectedMicID = "cs.selectedMicID"
     }
 
     private func loadSettings() {
@@ -73,6 +76,7 @@ final class AppState {
         captureMic = d.bool(forKey: Keys.captureMic)
         captureCamera = d.bool(forKey: Keys.captureCamera)
         selectedCameraID = d.string(forKey: Keys.selectedCameraID)
+        selectedMicID = d.string(forKey: Keys.selectedMicID)
     }
 
     func saveProjectsDirectory(_ url: URL) {
@@ -115,6 +119,15 @@ final class AppState {
         }
     }
 
+    func saveSelectedMicID(_ id: String?) {
+        selectedMicID = id
+        if let id = id {
+            UserDefaults.standard.set(id, forKey: Keys.selectedMicID)
+        } else {
+            UserDefaults.standard.removeObject(forKey: Keys.selectedMicID)
+        }
+    }
+
     // MARK: - Permissions
 
     func refreshPermissions() {

CineScreen/Capture/ScreenCaptureService.swift

@@ -17,6 +17,8 @@ struct CaptureRequest {
     var quality: Quality = .medium
     var captureSystemAudio: Bool = false
     var captureMic: Bool = false
+    /// AVCaptureDevice uniqueID for the microphone. `nil` = system default.
+    var micDeviceID: String? = nil
     var captureCamera: Bool = false
     /// AVCaptureDevice uniqueID. `nil` = system default camera.
     var cameraDeviceID: String? = nil
@@ -343,7 +345,7 @@ final class ScreenCaptureService: NSObject {
             }
             writer.add(m)
             mInput = m
-            (session, micDelegate) = try Self.makeMicSession(target: m, queue: micQueue, getSessionStart: { [weak self] in
+            (session, micDelegate) = try Self.makeMicSession(target: m, queue: micQueue, deviceID: request.micDeviceID, getSessionStart: { [weak self] in
                 self?.streamOutput?.sessionStartTime
             })
         }
@@ -575,14 +577,25 @@ final class ScreenCaptureService: NSObject {
         ]
     }
 
+    /// Discoverable microphones for the Settings picker.
+    static func availableMicrophones() -> [AVCaptureDevice] {
+        AVCaptureDevice.DiscoverySession(
+            deviceTypes: [.microphone],
+            mediaType: .audio,
+            position: .unspecified
+        ).devices
+    }
+
     private static func makeMicSession(
         target: AVAssetWriterInput,
         queue: DispatchQueue,
+        deviceID: String?,
         getSessionStart: @escaping () -> CMTime?
     ) throws -> (AVCaptureSession, MicOutputDelegate) {
         let session = AVCaptureSession()
         session.beginConfiguration()
-        guard let device = AVCaptureDevice.default(for: .audio) else {
+        let picked = deviceID.flatMap { AVCaptureDevice(uniqueID: $0) }
+        guard let device = picked ?? AVCaptureDevice.default(for: .audio) else {
             throw CaptureError.missingMicDevice
         }
         let input: AVCaptureDeviceInput

CineScreen/ControlBar/ControlBarView.swift

@@ -277,6 +277,7 @@ struct ControlBarView: View {
             quality: state.quality,
             captureSystemAudio: state.captureSystemAudio,
             captureMic: state.captureMic && state.permissions.microphone == .granted,
+            micDeviceID: state.selectedMicID,
             captureCamera: state.captureCamera && state.permissions.camera == .granted,
             cameraDeviceID: state.selectedCameraID,
             webcamURL: project.fallbackWebcamURL

CineScreen/MainWindow/SettingsView.swift

@@ -108,6 +108,16 @@ struct SettingsView: View {
                     .onChange(of: state.captureMic) { _, new in
                         state.saveCaptureMic(new)
                     }
+                Picker("Microphone", selection: Binding(
+                    get: { state.selectedMicID ?? "" },
+                    set: { state.saveSelectedMicID($0.isEmpty ? nil : $0) }
+                )) {
+                    Text("System default").tag("")
+                    ForEach(ScreenCaptureService.availableMicrophones(), id: \.uniqueID) { device in
+                        Text(device.localizedName).tag(device.uniqueID)
+                    }
+                }
+                .disabled(state.permissions.microphone != .granted)
             }
             Section("Webcam") {
                 Toggle("Record webcam by default", isOn: $state.captureCamera)