feat(client): 关于→关于我们，移到左侧菜单独立页面

- 新增 screens/about/about_screen.dart（版本信息/授权信息/关于我们/意见反馈）
- 左侧导航新增「关于我们」(/about)，从系统设置移除原「关于」Tab（7→6）
- 迁移并清理 settings 中仅关于页使用的 widget/弹窗/import

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/core/router/app_router.dart

@@ -15,6 +15,7 @@ import '../../screens/products/products_screen.dart';
 import '../../screens/products/product_detail_screen.dart';
 import '../../screens/public/public_product_screen.dart';
 import '../../screens/settings/settings_screen.dart';
+import '../../screens/about/about_screen.dart';
 import '../auth/auth_state.dart';
 
 Page<void> _noTransition(Widget child) =>
@@ -131,6 +132,9 @@ final appRouterProvider = Provider<GoRouter>((ref) {
           GoRoute(
               path: '/settings',
               pageBuilder: (_, __) => _noTransition(const SettingsScreen())),
+          GoRoute(
+              path: '/about',
+              pageBuilder: (_, __) => _noTransition(const AboutScreen())),
         ],
       ),
     ],