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/App.tsx

@@ -96,6 +96,9 @@ function AppContent() {
     toggleCamera,
     toggleMic,
     sendTextMessage,
+    cameras,
+    mics,
+    switchDevice,
   } = useVisionSession(accessToken, activeSessionId);
 
   const isConnected = connectionStatus === "connected";
@@ -406,21 +409,6 @@ function AppContent() {
                 <button className="btn btn--primary btn--lg" onClick={startSession}>
                   {connectionStatus === "connecting" ? tr("controls.connecting") : tr("controls.startVideo")}
                 </button>
-                {/* 设备选择器 */}
-                <div className="video-controls__devices">
-                  <div className="device-select-wrapper">
-                    <label className="device-select-label">📷 {tr("controls.device.camera")}</label>
-                    <select className="device-select" defaultValue="default">
-                      <option value="default">{tr("controls.device.default")}</option>
-                    </select>
-                  </div>
-                  <div className="device-select-wrapper">
-                    <label className="device-select-label">🎤 {tr("controls.device.mic")}</label>
-                    <select className="device-select" defaultValue="default">
-                      <option value="default">{tr("controls.device.default")}</option>
-                    </select>
-                  </div>
-                </div>
               </>
             ) : isCameraOn ? (
               <>
@@ -457,6 +445,35 @@ function AppContent() {
                     {tr("controls.stopVideo")}
                   </button>
                 </div>
+                {/* 设备选择器 */}
+                <div className="video-controls__devices">
+                  <div className="device-select-wrapper">
+                    <label className="device-select-label">📷</label>
+                    <select
+                      className="device-select"
+                      value={config.cameraDeviceId || "default"}
+                      onChange={(e) => switchDevice("camera", e.target.value === "default" ? "" : e.target.value)}
+                    >
+                      <option value="default">{tr("controls.device.default")}</option>
+                      {cameras.map((d) => (
+                        <option key={d.deviceId} value={d.deviceId}>{d.label}</option>
+                      ))}
+                    </select>
+                  </div>
+                  <div className="device-select-wrapper">
+                    <label className="device-select-label">🎤</label>
+                    <select
+                      className="device-select"
+                      value={config.micDeviceId || "default"}
+                      onChange={(e) => switchDevice("mic", e.target.value === "default" ? "" : e.target.value)}
+                    >
+                      <option value="default">{tr("controls.device.default")}</option>
+                      {mics.map((d) => (
+                        <option key={d.deviceId} value={d.deviceId}>{d.label}</option>
+                      ))}
+                    </select>
+                  </div>
+                </div>
               </>
             ) : (
               <>