camera: macOS device discovery now includes external + Continuity

Discovery was hard-coded to `.builtInWideAngleCamera` only — that
catches the iOS front/back cameras and the macOS FaceTime HD on
macOS 14+, but missed USB webcams (`.external` on 14+,
`.externalUnknown` before) and iPhone-as-webcam Continuity Camera
(`.continuityCamera` on 14+). On Macs whose built-in camera doesn't
expose itself as `.builtInWideAngleCamera`, the result was "no
camera detected".

Device types are now platform-conditional: iOS keeps the wide-angle
filter as-is; macOS adds the externals + Continuity (gated via
`if #available(macOS 14.0, *)` for the post-14 forms vs the
deprecated `.externalUnknown` for older macOS). The `#if os(macOS)`
guard is unavoidable — `.external` and friends literally aren't
declared as enum cases on iOS.

darwin/Camera/CaptureDevice.swift

@@ -8,18 +8,20 @@ import AVFoundation
 /// `TripleCamera` / `DualCamera` is for multi-cam zoom UX we don't
 /// build today; if we ever need it, this is where it goes.
 enum CaptureDevice {
-    /// Enumerate the front + back wide-angle cameras the system
+    /// Enumerate the cameras the system exposes. On iOS that's the
-    /// exposes. Order: back first, front second. Stable for the
+    /// front + back wide-angle cameras (in that order, back first).
-    /// lifetime of the process — iOS doesn't hot-swap cameras.
+    /// On macOS it's the built-in FaceTime HD plus any external /
+    /// Continuity cameras the OS reports.
     static func discover() -> [DiscoveredCamera] {
         let session = AVCaptureDevice.DiscoverySession(
-            deviceTypes: [.builtInWideAngleCamera],
+            deviceTypes: deviceTypes,
             mediaType: .video,
             position: .unspecified
         )
         // Sort so back devices come first; the chat composer opens
         // the back camera by default elsewhere, so this matches the
-        // common "first available" pick.
+        // common "first available" pick. On macOS positions are all
+        // `.unspecified`, so the sort is a no-op there.
         return session.devices
             .sorted { positionRank($0.position) < positionRank($1.position) }
             .map { device in
@@ -29,12 +31,34 @@ enum CaptureDevice {
                     // iOS doesn't expose sensor orientation directly;
                     // 90° matches what `camera_avfoundation` reports
                     // and what banlu's `normalizeCameraCapture` math
-                    // assumes for iOS sensors.
+                    // assumes for iOS sensors. macOS desktop cameras
+                    // are already landscape — 0 is the right answer
+                    // there but the value is unused on macOS (no
+                    // recording rotation, no preview rotation).
                     sensorOrientation: 90
                 )
             }
     }
 
+    /// Per-platform set of device types the discovery session asks
+    /// for. iOS only has the built-in cameras; macOS additionally
+    /// surfaces externals + Continuity Camera (iPhone-as-webcam).
+    /// `.external` / `.externalUnknown` / `.continuityCamera` are
+    /// macOS-only enum cases — referencing them on iOS would not
+    /// compile, hence the `#if os(macOS)` block.
+    private static var deviceTypes: [AVCaptureDevice.DeviceType] {
+        var types: [AVCaptureDevice.DeviceType] = [.builtInWideAngleCamera]
+        #if os(macOS)
+        if #available(macOS 14.0, *) {
+            types.append(.external)
+            types.append(.continuityCamera)
+        } else {
+            types.append(.externalUnknown)
+        }
+        #endif
+        return types
+    }
+
     /// Apply our default per-device config: continuous autofocus,
     /// continuous auto-exposure, torch off. Idempotent; safe to call
     /// repeatedly. The block is wrapped in