feat(mobile): login, password change and logout screens

P3 Task 3 移动端（对接 Task 2 api client + 本任务新增的后端契约）：

- 登录页实装：账号/密码表单（空值客户端校验）→ api.login（会话自动
  持久化）→ pushReplacement /home；错误码本地化（AUTH_INVALID_
  CREDENTIALS / AUTH_RATE_LIMITED（含稍后再试文案）/ 网络 / 校验）；
  提交加载态防重复提交。
- 修改密码页（设置页入口）：当前/新/确认三字段，客户端校验（≥8 位、
  两次一致）→ api.changePassword → toast + pop；成功后本地会话不变
  （服务端保留当前会话）；旧密码错误（401 AUTH_INVALID_CREDENTIALS）
  显示「当前密码错误」且不触发 401 驱逐。
- 退出登录：设置页按钮 → api.logout 尽力而为 → 清会话 → 清栈回
  /login；App 壳 initState 注册 onUnauthorized（业务 401 → 清栈 /login），
  根 navigatorKey 接线；password-change 的 401 豁免会话驱逐（拦截器
  豁免集合与令牌附加分离）。
- l10n：新增 16 组 zh/en 文案；错误码 → 文案解析扩展
  （core/error_l10n.dart，errorCode→AppLocalizations）。
- 测试：ApiClient.changePassword 单测 ×2（Bearer+请求体；旧密码 401
  不驱逐）；widget 测试 ×12（登录 5：空值/成功跳转/凭据错误/限速文案/
  加载禁用；改密 4：过短/不一致/旧密码错误文案+会话保留/成功 toast+pop
  且令牌不变；设置 2：改密入口跳转/登出清会话回登录；App 壳 1：401
  清栈回 /login）。flutter analyze 0 issue；flutter test 81/81。

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

apps/mobile/lib/l10n/app_localizations.dart

@@ -223,6 +223,102 @@ abstract class AppLocalizations {
   /// In zh, this message translates to:
   /// **'使用工作人员账号登录'**
   String get loginHint;
+
+  /// No description provided for @fieldRequired.
+  ///
+  /// In zh, this message translates to:
+  /// **'此项为必填项'**
+  String get fieldRequired;
+
+  /// No description provided for @passwordChangeTitle.
+  ///
+  /// In zh, this message translates to:
+  /// **'修改密码'**
+  String get passwordChangeTitle;
+
+  /// No description provided for @currentPassword.
+  ///
+  /// In zh, this message translates to:
+  /// **'当前密码'**
+  String get currentPassword;
+
+  /// No description provided for @newPassword.
+  ///
+  /// In zh, this message translates to:
+  /// **'新密码'**
+  String get newPassword;
+
+  /// No description provided for @confirmNewPassword.
+  ///
+  /// In zh, this message translates to:
+  /// **'确认新密码'**
+  String get confirmNewPassword;
+
+  /// No description provided for @passwordMinLength.
+  ///
+  /// In zh, this message translates to:
+  /// **'新密码至少需要 8 个字符'**
+  String get passwordMinLength;
+
+  /// No description provided for @passwordMismatch.
+  ///
+  /// In zh, this message translates to:
+  /// **'两次输入的新密码不一致'**
+  String get passwordMismatch;
+
+  /// No description provided for @passwordChangeSuccess.
+  ///
+  /// In zh, this message translates to:
+  /// **'密码已修改'**
+  String get passwordChangeSuccess;
+
+  /// No description provided for @errorAuthInvalidCredentials.
+  ///
+  /// In zh, this message translates to:
+  /// **'账号或密码错误'**
+  String get errorAuthInvalidCredentials;
+
+  /// No description provided for @errorOldPasswordIncorrect.
+  ///
+  /// In zh, this message translates to:
+  /// **'当前密码错误'**
+  String get errorOldPasswordIncorrect;
+
+  /// No description provided for @errorAuthRateLimited.
+  ///
+  /// In zh, this message translates to:
+  /// **'尝试次数过多，请稍后再试'**
+  String get errorAuthRateLimited;
+
+  /// No description provided for @errorAuthTokenExpired.
+  ///
+  /// In zh, this message translates to:
+  /// **'会话已过期，请重新登录'**
+  String get errorAuthTokenExpired;
+
+  /// No description provided for @errorValidation.
+  ///
+  /// In zh, this message translates to:
+  /// **'输入有误，请检查后重试'**
+  String get errorValidation;
+
+  /// No description provided for @errorNetwork.
+  ///
+  /// In zh, this message translates to:
+  /// **'网络连接失败，请检查网络后重试'**
+  String get errorNetwork;
+
+  /// No description provided for @errorNetworkTimeout.
+  ///
+  /// In zh, this message translates to:
+  /// **'网络超时，请重试'**
+  String get errorNetworkTimeout;
+
+  /// No description provided for @errorInternal.
+  ///
+  /// In zh, this message translates to:
+  /// **'服务暂时不可用，请稍后重试'**
+  String get errorInternal;
 }
 
 class _AppLocalizationsDelegate