feat: 认证失败时停止重连并提示用户

密码错误、账户禁用、令牌过期或限流等永久性认证错误不再无限
重连，改为置 StateError 并通过 EvError 弹窗告知用户具体原因
（中英文本地化）。网络错误仍正常指数退避重试。

- protocol: 新增 AuthErrorCode 类型与消息映射
- transport: AuthError/ServerError 携带 Code 字段
- vpn: run() 循环识别致命认证错误并停止，新增 onError 回调
- ipc/daemon/ui: 打通错误码到 UI 的本地化展示

internal/i18n/en.toml

@@ -53,6 +53,12 @@ DlgVPNError = "VPN Error"
 DlgSaveError = "Save Error"
 DlgKeychainError = "Keychain Error"
 DlgError = "Error"
+DlgAuthError = "Authentication Failed"
+AuthErrWrongCredentials = "Wrong username or password. Please check your credentials."
+AuthErrUserDisabled = "This user account does not exist or has been disabled."
+AuthErrTokenInvalid = "The authentication token is invalid or expired."
+AuthErrRateLimited = "Too many authentication attempts. Please try again later."
+AuthErrMalformed = "Authentication message format error."
 
 BtnResetDB = "Reset Database"
 DlgResetDBTitle = "Reset Database"