feat: 实现前端认证系统，WebSocket 连接携带 JWT token

- 新增 AuthContext/AuthProvider：登录/注册状态管理，JWT 自动刷新
- 新增 AuthPage 组件：登录/注册表单，支持模式切换和前端校验
- 新增 api.ts：封装 auth REST API 客户端（register/login/refresh/logout）
- WebSocket 连接时拼接 ?token=<jwt>，重连自动携带
- useVisionSession 接受 accessToken 参数并透传
- App.tsx 包裹 AuthProvider，未登录时显示登录页
- storage.ts 新增 token/user 的 localStorage 存储
- i18n 新增中/英/日三语 auth 翻译
- App.css 新增 auth 页面和用户 badge 样式

frontend/src/lib/api.ts

@@ -0,0 +1,106 @@
+// ============================================================
+// HTTP API 客户端
+// 职责：封装 REST API 请求（auth、conversations 等）
+// ============================================================
+
+const API_BASE = "/api";
+
+interface ApiResponse<T> {
+  data?: T;
+  error?: { code: string; message: string };
+  status: number;
+}
+
+async function request<T>(
+  path: string,
+  options: RequestInit = {}
+): Promise<ApiResponse<T>> {
+  const url = `${API_BASE}${path}`;
+  const headers: Record<string, string> = {
+    "Content-Type": "application/json",
+    ...(options.headers as Record<string, string>),
+  };
+
+  try {
+    const res = await fetch(url, { ...options, headers });
+    const status = res.status;
+
+    if (res.status === 204) {
+      return { status };
+    }
+
+    const body = await res.json();
+
+    if (!res.ok) {
+      return {
+        error: { code: body.error || "UNKNOWN", message: body.message || "请求失败" },
+        status,
+      };
+    }
+
+    return { data: body as T, status };
+  } catch (err) {
+    return {
+      error: { code: "NETWORK_ERROR", message: "网络连接失败，请检查网络" },
+      status: 0,
+    };
+  }
+}
+
+function authHeaders(accessToken: string): Record<string, string> {
+  return { Authorization: `Bearer ${accessToken}` };
+}
+
+// ---- Auth API ----
+
+export interface AuthUser {
+  id: string;
+  username: string;
+  created_at: string;
+}
+
+export interface AuthResponse {
+  user: AuthUser;
+  access_token: string;
+  refresh_token: string;
+}
+
+export async function register(
+  username: string,
+  password: string
+): Promise<ApiResponse<AuthResponse>> {
+  return request<AuthResponse>("/auth/register", {
+    method: "POST",
+    body: JSON.stringify({ username, password }),
+  });
+}
+
+export async function login(
+  username: string,
+  password: string
+): Promise<ApiResponse<AuthResponse>> {
+  return request<AuthResponse>("/auth/login", {
+    method: "POST",
+    body: JSON.stringify({ username, password }),
+  });
+}
+
+export async function refreshToken(
+  refresh_token: string
+): Promise<ApiResponse<AuthResponse>> {
+  return request<AuthResponse>("/auth/refresh", {
+    method: "POST",
+    body: JSON.stringify({ refresh_token }),
+  });
+}
+
+export async function logout(
+  accessToken: string,
+  refreshToken: string
+): Promise<ApiResponse<{ message: string }>> {
+  return request<{ message: string }>("/auth/logout", {
+    method: "POST",
+    headers: authHeaders(accessToken),
+    body: JSON.stringify({ refresh_token: refreshToken }),
+  });
+}