feat(auth): 添加登录/注册页面及令牌恢复流程

- 新增 SplashPage（启动页）和 LoginPage（登录页），支持路由跳转
- 扩展 AuthState，增加 submitting/alert/busy 状态及消费机制
- AuthController 增加 checkLoginState/register/showAlert 等方法
- 删除旧模型文件（binding_item, token_response, verify_response）
- 删除旧版 LoginPage，迁移至 features/auth 目录
- 更新本地化字符串，增加注册相关文案
- 调整默认 API 地址为本地开发环境

webrtc_controller_flutter/lib/l10n/app_en.arb

@@ -3,6 +3,10 @@
   "appTitle": "WebRTC Controller",
   "login": "Login",
   "logout": "Logout",
+  "registerAccount": "Create Account",
+  "registerSuccess": "Registered successfully, please log in",
+  "registerFailed": "Registration failed: {error}",
+  "loginSubtitle": "Log in to connect your bound devices",
   "cancel": "Cancel",
   "confirm": "OK",
   "connect": "Connect",