feat: 集成 VAD 语音活动检测，打通核心交互链路

- 重写 useVAD Hook，接入 @ricky0123/vad-web 的 MicVAD
- 支持 onSpeechStart/onSpeechEnd/onVADMisfire 回调
- useVisionSession 中串联摄像头→麦克风→WebSocket→VAD 完整流程
- App.tsx 新增 VAD 加载中和错误状态的 UI 指示
- VAD 参数对齐设计文档：positiveSpeechThreshold=0.5, minSpeechMs=250

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/hooks/useVisionSession.ts

@@ -9,6 +9,7 @@ import { v4 as uuidv4 } from "uuid";
 import { wsClient } from "../lib/websocket";
 import { encodeAudioToBase64, dataUrlToBase64 } from "../lib/audio";
 import { useCamera } from "../components/CameraManager";
+import { useMicrophone } from "../components/MicManager";
 import { useVAD } from "../components/EdgeProcessor";
 import { useWebSocketManager } from "../components/WebSocketManager";
 import type { ChatMessage, ServerMessage, LLMDoneMessage } from "../types";
@@ -19,10 +20,17 @@ export function useVisionSession() {
   const [isProcessing, setIsProcessing] = useState(false);
 
   const { videoRef, captureFrame, startCamera, stopCamera, stream } = useCamera();
+  const { startMic, stopMic } = useMicrophone();
   const { status, connect, disconnect, send } = useWebSocketManager();
 
   // VAD：语音结束时自动发送 query
-  const { isSpeaking, start: startVAD, stop: stopVAD } = useVAD({
+  const {
+    isSpeaking,
+    isReady: isVADReady,
+    error: vadError,
+    start: startVAD,
+    stop: stopVAD,
+  } = useVAD({
     onSpeechEnd: useCallback(
       (audio: Float32Array) => {
         const frame = captureFrame();
@@ -101,17 +109,28 @@ export function useVisionSession() {
 
   /** 启动会话 */
   const startSession = useCallback(async () => {
+    // 1. 获取摄像头和麦克风
     await startCamera();
+    const micStream = await startMic();
+    if (!micStream) {
+      console.error("[Session] 无法获取麦克风");
+      return;
+    }
+
+    // 2. 连接 WebSocket
     connect();
-    startVAD();
-  }, [startCamera, connect, startVAD]);
+
+    // 3. 启动 VAD（传入麦克风 stream）
+    await startVAD(micStream);
+  }, [startCamera, startMic, connect, startVAD]);
 
   /** 结束会话 */
-  const stopSession = useCallback(() => {
-    stopVAD();
+  const stopSession = useCallback(async () => {
+    await stopVAD();
+    stopMic();
     stopCamera();
     disconnect();
-  }, [stopVAD, stopCamera, disconnect]);
+  }, [stopVAD, stopMic, stopCamera, disconnect]);
 
   /** 打断当前回复 */
   const interrupt = useCallback(() => {
@@ -124,6 +143,8 @@ export function useVisionSession() {
     currentReply,
     isProcessing,
     isSpeaking,
+    isVADReady,
+    vadError,
     connectionStatus: status,
     videoRef,
     stream,