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/lib/types.ts

@@ -100,6 +100,7 @@ export interface AppSettings {
   auto_check_updates: boolean;
   dismissed_update_version: string | null;
   timezone: string | null;
+  default_microphone: string | null;
 }
 
 export interface UpdateInfo {