feat(app): 设置页 UI + Mood/成就/贴纸 BLoC 接入 API + B7 测试扩展

前端改动:
- 新建设置页面 (主题切换/关于/隐私政策/用户协议/儿童隐私保护)
- SettingsBloc 注册到 MultiRepositoryProvider 全局可访问
- MoodBloc 修复编译错误 + 接入 /diary/stats/mood API
- MoodPage 添加错误状态展示和重试按钮
- AchievementBloc + 页面改造接入 /diary/achievements API
- StickerBloc + 页面改造接入 /diary/sticker-packs API
- TemplateBloc + 页面改造接入 /diary/templates API
- ProfilePage 设置入口改为跳转 /settings
- 添加 /settings 路由

后端改动:
- 扩展 mood_stats_service 测试 (连续天数算法/心情计数/边界场景)
- 新增 class_service 测试 (班级码生成/唯一性/错误映射)
- 新增 achievement_service 测试 (DTO 结构/序列化/map 构建)
- 新增 sticker_service 测试 (DTO 序列化/错误处理)
- 扩展 dto.rs 测试 (achievement/mood_stats/sticker/template/notification)
- 清理 2 个 unused import warning

验证:
- cargo check 0 error 0 warning
- flutter analyze 0 error

app/lib/core/routing/app_router.dart

@@ -30,6 +30,7 @@ import '../../features/parent/views/parent_page.dart';
 import '../../features/achievement/views/achievement_page.dart';
 import '../../features/stickers/views/sticker_library_page.dart';
 import '../../features/templates/views/template_gallery_page.dart';
+import '../../features/settings/views/settings_page.dart';
 import '../../features/auth/bloc/auth_bloc.dart';
 
 // Shell 分支键
@@ -192,6 +193,12 @@ GoRouter createAppRouter(AuthBloc authBloc) {
         parentNavigatorKey: _rootNavigatorKey,
         builder: (context, state) => const TemplateGalleryPage(),
       ),
+      GoRoute(
+        path: '/settings',
+        name: 'settings',
+        parentNavigatorKey: _rootNavigatorKey,
+        builder: (context, state) => const SettingsPage(),
+      ),
     ],
   );
 }