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-tauri/src/models/app_settings.rs

@@ -70,6 +70,8 @@ pub struct AppSettings {
     pub dismissed_update_version: Option<String>,
     #[serde(default)]
     pub timezone: Option<String>,
+    #[serde(default)]
+    pub default_microphone: Option<String>,
 }
 
 impl Default for AppSettings {
@@ -87,6 +89,7 @@ impl Default for AppSettings {
             auto_check_updates: true,
             dismissed_update_version: None,
             timezone: None,
+            default_microphone: None,
         }
     }
 }