feat: add microphone selection to settings

Adds a dropdown in Settings to choose which audio input device to
use for voice mode. Enumerates devices via the browser's
mediaDevices API and persists the selection in AppSettings.
The useVoice hook passes the selected deviceId to getUserMedia().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src/hooks/useVoice.ts

@@ -3,7 +3,7 @@ import * as commands from "../lib/tauri-commands";
 
 type VoiceState = "inactive" | "starting" | "active" | "error";
 
-export function useVoice(sessionId: string) {
+export function useVoice(sessionId: string, deviceId?: string | null) {
   const [state, setState] = useState<VoiceState>("inactive");
   const [error, setError] = useState<string | null>(null);
 
@@ -20,14 +20,19 @@ export function useVoice(sessionId: string) {
       // 1. Start the audio bridge in the container (creates FIFO writer)
       await commands.startAudioBridge(sessionId);
 
-      // 2. Get microphone access
+      // 2. Get microphone access (use specific device if configured)
+      const audioConstraints: MediaTrackConstraints = {
+        channelCount: 1,
+        echoCancellation: true,
+        noiseSuppression: true,
+        autoGainControl: true,
+      };
+      if (deviceId) {
+        audioConstraints.deviceId = { exact: deviceId };
+      }
+
       const stream = await navigator.mediaDevices.getUserMedia({
-        audio: {
-          channelCount: 1,
-          echoCancellation: true,
-          noiseSuppression: true,
-          autoGainControl: true,
-        },
+        audio: audioConstraints,
       });
       streamRef.current = stream;
 
@@ -62,7 +67,7 @@ export function useVoice(sessionId: string) {
       // Clean up on failure
       await commands.stopAudioBridge(sessionId).catch(() => {});
     }
-  }, [sessionId, state]);
+  }, [sessionId, state, deviceId]);
 
   const stop = useCallback(async () => {
     // Tear down audio pipeline