feat: 实现摄像头和麦克风设备选择功能

- 新增 useDeviceList hook，枚举音视频输入设备并监听热插拔
- CameraManager/MicManager 的 startCamera/startMic 支持可选 deviceId 参数
- useVisionSession 集成设备选择：授权后自动枚举、切换设备时热重启
- 连接后显示设备下拉选择器，未连接时隐藏（避免未授权时空列表）
- SessionConfig 新增 cameraDeviceId/micDeviceId 持久化到 localStorage

frontend/src/components/CameraManager/index.tsx

@@ -19,10 +19,13 @@ export function useCamera() {
   const [stream, setStream] = useState<MediaStream | null>(null);
   const [error, setError] = useState<string | null>(null);
 
-  const startCamera = useCallback(async () => {
+  const startCamera = useCallback(async (deviceId?: string) => {
     try {
+      const videoConstraints: MediaTrackConstraints = deviceId
+        ? { deviceId: { exact: deviceId }, width: 640, height: 480 }
+        : { facingMode: "environment", width: 640, height: 480 };
       const mediaStream = await navigator.mediaDevices.getUserMedia({
-        video: { facingMode: "environment", width: 640, height: 480 },
+        video: videoConstraints,
         audio: false,
       });
       setStream(mediaStream);