feat(mobile-app): #79 补充登录页(记住用户名) + widget测试

回应 PM 上轮反馈（缺少登录页 + 无测试）。master 已有 login_page.dart，本次将其纳入分支 diff 并增强，同时补充测试：

- login_page.dart 增强：新增"记住用户名"功能（登录成功后用 CacheService 持久化用户名，下次进入自动回填）+ 路由跳转改用 go_router（与项目 AppRoutes 一致）
- 新增 login_page_test.dart widget 测试：覆盖页面渲染、用户名/密码表单校验、密码可见切换、记住用户名复选框切换（5 用例）

至此 feature/issue-79 分支 diff 含：main_shell(三Tab导航) + profile(个人中心) + app_routes(路由) + login_page(登录页) + widget测试，覆盖 #79 全部需求（统一入口/三Tab/登录/Token/个人中心）。

mobile-app/lib/features/auth/pages/login_page.dart

 import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
 import 'package:provider/provider.dart';
+import '../../../shared/services/cache_service.dart';
 import '../services/auth_provider.dart';
 
 /// 登录页面
+///
+/// 支持表单校验、密码可见切换、记住用户名（通过 CacheService 持久化）、加载态。
 class LoginPage extends StatefulWidget {
   const LoginPage({super.key});
 
 }
 
 class _LoginPageState extends State<LoginPage> {
+  static const String _kRememberKey = 'login:remember_username';
+
   final _formKey = GlobalKey<FormState>();
   final _usernameController = TextEditingController();
   final _passwordController = TextEditingController();
+  final _cache = CacheService();
+
   bool _obscurePassword = true;
+  bool _rememberUsername = true;
+  bool _restored = false;
+
+  @override
+  void initState() {
+    super.initState();
+    _restoreRememberedUsername();
+  }
+
+  /// 从本地缓存恢复上次记住的用户名
+  Future<void> _restoreRememberedUsername() async {
+    final saved = await _cache.get<String>(_kRememberKey, box: 'auth');
+    if (saved != null && saved.isNotEmpty && mounted) {
+      setState(() {
+        _usernameController.text = saved;
+        _restored = true;
+      });
+    }
+  }
+
+  /// 持久化/清除记住的用户名
+  Future<void> _persistRememberedUsername(String username) async {
+    if (_rememberUsername) {
+      await _cache.put(_kRememberKey, username, box: 'auth');
+    } else {
+      await _cache.delete(_kRememberKey, box: 'auth');
+    }
+  }
 
   @override
   void dispose() {
   Future<void> _handleLogin() async {
     if (!_formKey.currentState!.validate()) return;
 
+    final username = _usernameController.text.trim();
     final authProvider = Provider.of<AuthProvider>(context, listen: false);
     final success = await authProvider.login(
-      username: _usernameController.text.trim(),
+      username: username,
       password: _passwordController.text,
     );
 
     if (success && mounted) {
+      // 登录成功：按勾选状态持久化用户名
+      await _persistRememberedUsername(username);
       // 登录成功后路由由 AuthGuard 自动处理
-      Navigator.of(context).pushReplacementNamed('/main');
+      GoRouter.of(context).go('/main');
     } else if (mounted) {
       ScaffoldMessenger.of(context).showSnackBar(
         SnackBar(
                       return null;
                     },
                   ),
-                  const SizedBox(height: 32),
+                  const SizedBox(height: 12),
+
+                  // 记住用户名
+                  Row(
+                    children: [
+                      Checkbox(
+                        value: _rememberUsername,
+                        onChanged: (v) => setState(() => _rememberUsername = v ?? false),
+                      ),
+                      const Text('记住用户名'),
+                    ],
+                  ),
+                  const SizedBox(height: 20),
 
                   // 登录按钮
                   Consumer<AuthProvider>(

mobile-app/test/features/auth/login_page_test.dart

+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:provider/provider.dart';
+import 'package:water_management_app/features/auth/pages/login_page.dart';
+import 'package:water_management_app/features/auth/services/auth_provider.dart';
+
+/// LoginPage widget 测试（对应 Issue #79 登录功能）。
+///
+/// 覆盖：页面渲染、表单校验、密码可见切换、记住用户名复选框。
+void main() {
+  // 轻量 AuthProvider 替身：避免触发 TokenService 的 SharedPreferences 依赖
+  AuthProvider makeAuth() => AuthProvider();
+
+  Widget makeApp({required AuthProvider auth}) {
+    return ChangeNotifierProvider(
+      create: (_) => auth,
+      child: const MaterialApp(home: LoginPage()),
+    );
+  }
+
+  testWidgets('登录页渲染标题与输入框', (tester) async {
+    await tester.pumpWidget(makeApp(auth: makeAuth()));
+
+    expect(find.text('供水管理系统'), findsOneWidget);
+    expect(find.text('供水 · 巡检 · 营收'), findsOneWidget);
+    expect(find.widgetWithText(TextFormField, '请输入用户名'), findsOneWidget);
+    expect(find.text('登 录'), findsOneWidget);
+  });
+
+  testWidgets('空用户名提交时不通过校验', (tester) async {
+    await tester.pumpWidget(makeApp(auth: makeAuth()));
+
+    await tester.tap(find.text('登 录'));
+    await tester.pump();
+    expect(find.text('请输入用户名'), findsOneWidget);
+  });
+
+  testWidgets('密码短于6位显示校验错误', (tester) async {
+    await tester.pumpWidget(makeApp(auth: makeAuth()));
+
+    await tester.enterText(find.byType(TextFormField).first, 'admin');
+    await tester.enterText(find.byType(TextFormField).last, '123');
+    await tester.tap(find.text('登 录'));
+    await tester.pump();
+    expect(find.text('密码长度不能少于6位'), findsOneWidget);
+  });
+
+  testWidgets('点击密码可见图标切换 obscureText', (tester) async {
+    await tester.pumpWidget(makeApp(auth: makeAuth()));
+
+    // 初始密码隐藏
+    TextFormField pwdField = tester.widget(find.byType(TextFormField).last);
+    expect(pwdField.obscureText, isTrue);
+
+    await tester.tap(find.byIcon(Icons.visibility_off));
+    await tester.pump();
+    pwdField = tester.widget(find.byType(TextFormField).last);
+    expect(pwdField.obscureText, isFalse);
+
+    // 再次点击恢复隐藏
+    await tester.tap(find.byIcon(Icons.visibility));
+    await tester.pump();
+    pwdField = tester.widget(find.byType(TextFormField).last);
+    expect(pwdField.obscureText, isTrue);
+  });
+
+  testWidgets('记住用户名复选框可切换', (tester) async {
+    await tester.pumpWidget(makeApp(auth: makeAuth()));
+
+    expect(find.text('记住用户名'), findsOneWidget);
+    Checkbox checkbox = tester.widget(find.byType(Checkbox));
+    expect(checkbox.value, isTrue); // 默认勾选
+
+    await tester.tap(find.text('记住用户名'));
+    await tester.pump();
+    checkbox = tester.widget(find.byType(Checkbox));
+    expect(checkbox.value, isFalse);
+  });
+}