feat: 实现 TTS 语音播放，完成 MVP P0 全部用户故事

- 新增 TTSPlayer：收集流式 tts_audio 片段，is_last 时拼接解码播放
- useVisionSession 接入 TTS 播放器，interrupt/stopSession 时停止播放
- App 显示 '🔊 正在播放...' 指示器
- MVP P0 四个用户故事前端全部实现（US-01 ~ US-04）

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 { getErrorMessage } from "../lib/errors";
+import { TTSPlayer } from "../lib/ttsPlayer";
 import { showToast } from "../lib/toast";
 import { useCamera } from "../components/CameraManager";
 import { useMicrophone } from "../components/MicManager";
@@ -20,6 +21,18 @@ export function useVisionSession() {
   const [messages, setMessages] = useState<ChatMessage[]>([]);
   const [currentReply, setCurrentReply] = useState<string>("");
   const [isProcessing, setIsProcessing] = useState(false);
+  const [isAudioPlaying, setIsAudioPlaying] = useState(false);
+
+  // TTS 播放器
+  const ttsPlayerRef = useRef<TTSPlayer | null>(null);
+  const getTTSPlayer = useCallback(() => {
+    if (!ttsPlayerRef.current) {
+      const player = new TTSPlayer();
+      player.onEnd(() => setIsAudioPlaying(false));
+      ttsPlayerRef.current = player;
+    }
+    return ttsPlayerRef.current;
+  }, []);
 
   const { videoRef, captureFrame, startCamera, stopCamera, stream } = useCamera();
   const { startMic, stopMic } = useMicrophone();
@@ -115,7 +128,10 @@ export function useVisionSession() {
         }
 
         case "tts_audio":
-          // TODO: 阶段 5 音频流播放
+          getTTSPlayer().enqueue(msg.audio, msg.mime_type, msg.is_last);
+          if (!msg.is_last) {
+            setIsAudioPlaying(true);
+          }
           break;
 
         case "error":
@@ -127,7 +143,7 @@ export function useVisionSession() {
     });
 
     return unsub;
-  }, []);
+  }, [getTTSPlayer]);
 
   // 连接断开时显示提示
   useEffect(() => {
@@ -160,7 +176,9 @@ export function useVisionSession() {
     stopMic();
     stopCamera();
     disconnect();
-    // 清理所有对话状态
+    // 停止 TTS 并清理状态
+    ttsPlayerRef.current?.stop();
+    setIsAudioPlaying(false);
     setMessages([]);
     setCurrentReply("");
     setIsProcessing(false);
@@ -169,6 +187,9 @@ export function useVisionSession() {
   /** 打断当前回复 */
   const interrupt = useCallback(() => {
     send({ type: "interrupt" });
+    // 停止 TTS 播放
+    ttsPlayerRef.current?.stop();
+    setIsAudioPlaying(false);
     // 将未完成的流式内容保存为最终消息
     if (currentReply) {
       setMessages((prev) => [
@@ -184,6 +205,7 @@ export function useVisionSession() {
     messages,
     currentReply,
     isProcessing,
+    isAudioPlaying,
     isSpeaking,
     isVADReady,
     vadError,