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/lib/ttsPlayer.ts

@@ -0,0 +1,104 @@
+// ============================================================
+// TTS Player — 语音播放器
+// 职责：收集后端流式 tts_audio 片段，拼接后播放
+// 格式：MVP 仅支持 audio/mp3，pcm 为 TODO
+// ============================================================
+
+type OnEndCallback = () => void;
+
+export class TTSPlayer {
+  private chunks: string[] = [];
+  private audio: HTMLAudioElement | null = null;
+  private _isPlaying = false;
+  private onEndCallback: OnEndCallback | null = null;
+
+  /** 注册播放完成回调 */
+  onEnd(cb: OnEndCallback): void {
+    this.onEndCallback = cb;
+  }
+
+  /** 当前是否正在播放 */
+  get isPlaying(): boolean {
+    return this._isPlaying;
+  }
+
+  /**
+   * 入队一个 TTS 音频片段
+   * @param base64 Base64 编码的音频数据
+   * @param mimeType 音频格式（"audio/mp3" 或 "audio/pcm"）
+   * @param isLast 是否为最后一个片段
+   */
+  enqueue(base64: string, mimeType: string, isLast: boolean): void {
+    this.chunks.push(base64);
+
+    if (isLast) {
+      this.play(mimeType);
+    }
+  }
+
+  /** 停止播放并清空缓冲区 */
+  stop(): void {
+    if (this.audio) {
+      this.audio.pause();
+      this.audio.removeAttribute("src");
+      this.audio = null;
+    }
+    this.chunks = [];
+    this._isPlaying = false;
+  }
+
+  /** 暂停播放 */
+  pause(): void {
+    this.audio?.pause();
+  }
+
+  /** 恢复播放 */
+  resume(): void {
+    this.audio?.play();
+  }
+
+  /** 拼接所有片段并播放 */
+  private play(mimeType: string): void {
+    if (this.chunks.length === 0) return;
+
+    // 拼接所有 Base64 片段
+    const combined = this.chunks.join("");
+    this.chunks = [];
+
+    // Base64 → Uint8Array → Blob
+    const binary = atob(combined);
+    const bytes = new Uint8Array(binary.length);
+    for (let i = 0; i < binary.length; i++) {
+      bytes[i] = binary.charCodeAt(i);
+    }
+    const blob = new Blob([bytes], { type: mimeType });
+    const url = URL.createObjectURL(blob);
+
+    // 播放
+    const audio = new Audio(url);
+    this.audio = audio;
+    this._isPlaying = true;
+
+    audio.onended = () => {
+      URL.revokeObjectURL(url);
+      this._isPlaying = false;
+      this.audio = null;
+      this.onEndCallback?.();
+    };
+
+    audio.onerror = () => {
+      console.error("[TTS] 播放失败");
+      URL.revokeObjectURL(url);
+      this._isPlaying = false;
+      this.audio = null;
+      this.onEndCallback?.();
+    };
+
+    audio.play().catch((err) => {
+      console.error("[TTS] play() 被拒绝:", err);
+      this._isPlaying = false;
+      this.audio = null;
+      this.onEndCallback?.();
+    });
+  }
+}