feat: 初始化前端项目，搭建 React + TypeScript + Vite 工程结构

- 创建 Vite + React 18 + TypeScript 项目（严格模式）
- 安装 @ricky0123/vad-web、onnxruntime-web、uuid 依赖
- 定义全部 WebSocket 消息类型和数据模型（对齐接口文档）
- 实现 WebSocket 连接管理（心跳保活、指数退避重连）
- 实现音频编码工具（PCM↔Base64、DataURL 转换）
- 创建组件骨架：CameraManager、MicManager、EdgeProcessor、WebSocketManager、ChatPanel、VideoPreview
- 实现核心 useVisionSession Hook 骨架
- 配置 ESLint（_前缀变量忽略规则）

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

frontend/src/components/CameraManager/index.tsx

@@ -0,0 +1,59 @@
+// ============================================================
+// CameraManager — 摄像头流采集
+// 职责：获取用户摄像头 MediaStream，提供给 VideoPreview 和 EdgeProcessor
+// ============================================================
+
+import { useCallback, useRef, useState } from "react";
+
+export interface CameraManagerHandle {
+  /** 获取当前视频轨道 */
+  stream: MediaStream | null;
+  /** 捕获当前帧为 JPEG DataURL */
+  captureFrame: () => string | null;
+}
+
+export function useCamera() {
+  const videoRef = useRef<HTMLVideoElement>(null);
+  const [stream, setStream] = useState<MediaStream | null>(null);
+  const [error, setError] = useState<string | null>(null);
+
+  const startCamera = useCallback(async () => {
+    try {
+      const mediaStream = await navigator.mediaDevices.getUserMedia({
+        video: { facingMode: "environment", width: 640, height: 480 },
+        audio: false,
+      });
+      setStream(mediaStream);
+      if (videoRef.current) {
+        videoRef.current.srcObject = mediaStream;
+      }
+      setError(null);
+    } catch (err) {
+      const message = err instanceof Error ? err.message : "无法访问摄像头";
+      setError(message);
+      console.error("[Camera] 获取摄像头失败:", err);
+    }
+  }, []);
+
+  const stopCamera = useCallback(() => {
+    stream?.getTracks().forEach((track) => track.stop());
+    setStream(null);
+  }, [stream]);
+
+  /** 从 video 元素捕获当前帧为 JPEG DataURL */
+  const captureFrame = useCallback((): string | null => {
+    const video = videoRef.current;
+    if (!video || video.readyState < 2) return null;
+
+    const canvas = document.createElement("canvas");
+    canvas.width = video.videoWidth;
+    canvas.height = video.videoHeight;
+    const ctx = canvas.getContext("2d");
+    if (!ctx) return null;
+
+    ctx.drawImage(video, 0, 0);
+    return canvas.toDataURL("image/jpeg", 0.7);
+  }, []);
+
+  return { videoRef, stream, error, startCamera, stopCamera, captureFrame };
+}