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/i18n/en-US.ts

@@ -130,4 +130,19 @@ export const enUS: TranslationMap = {
   "error.vadInit": "VAD initialization failed",
   "error.cameraAccess": "Cannot access camera",
   "error.micAccess": "Cannot access microphone",
+
+  // Auth
+  "auth.subtitle": "AI Vision Assistant",
+  "auth.login": "Sign In",
+  "auth.register": "Sign Up",
+  "auth.username": "Username",
+  "auth.username.placeholder": "3-64 characters",
+  "auth.password": "Password",
+  "auth.password.placeholder": "8-72 characters",
+  "auth.submitting": "Please wait...",
+  "auth.noAccount": "Don't have an account?",
+  "auth.hasAccount": "Already have an account?",
+  "auth.error.usernameLength": "Username must be 3-64 characters",
+  "auth.error.passwordLength": "Password must be 8-72 characters",
+  "auth.logout": "Sign Out",
 };