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/components/settings/SettingsPanel.tsx

@@ -2,6 +2,7 @@ import { useState, useEffect } from "react";
 import ApiKeyInput from "./ApiKeyInput";
 import DockerSettings from "./DockerSettings";
 import AwsSettings from "./AwsSettings";
+import MicrophoneSettings from "./MicrophoneSettings";
 import { useSettings } from "../../hooks/useSettings";
 import { useUpdates } from "../../hooks/useUpdates";
 import ClaudeInstructionsModal from "../projects/ClaudeInstructionsModal";
@@ -59,6 +60,8 @@ export default function SettingsPanel() {
       <DockerSettings />
       <AwsSettings />
 
+      <MicrophoneSettings />
+
       {/* Container Timezone */}
       <div>
         <label className="block text-sm font-medium mb-1">Container Timezone</label>