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_en.dart

@@ -70,4 +70,54 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get loginHint => 'Sign in with your staff account';
+
+  @override
+  String get fieldRequired => 'This field is required';
+
+  @override
+  String get passwordChangeTitle => 'Change password';
+
+  @override
+  String get currentPassword => 'Current password';
+
+  @override
+  String get newPassword => 'New password';
+
+  @override
+  String get confirmNewPassword => 'Confirm new password';
+
+  @override
+  String get passwordMinLength => 'New password must be at least 8 characters';
+
+  @override
+  String get passwordMismatch => 'New passwords do not match';
+
+  @override
+  String get passwordChangeSuccess => 'Password changed';
+
+  @override
+  String get errorAuthInvalidCredentials => 'Incorrect account or password';
+
+  @override
+  String get errorOldPasswordIncorrect => 'Current password is incorrect';
+
+  @override
+  String get errorAuthRateLimited =>
+      'Too many attempts. Please try again later';
+
+  @override
+  String get errorAuthTokenExpired => 'Session expired. Please sign in again';
+
+  @override
+  String get errorValidation => 'Invalid input. Please check and retry';
+
+  @override
+  String get errorNetwork => 'Network error. Check your connection and retry';
+
+  @override
+  String get errorNetworkTimeout => 'Network timed out. Please retry';
+
+  @override
+  String get errorInternal =>
+      'Service temporarily unavailable. Please retry later';
 }