refactor(stores): unify data sources pattern and extract BaseManager base class

- Add BaseManager/CachedManager base classes to eliminate duplicate cache logic
- Add postListSources.ts with IPostListPagedSource interface
- Add groupListSources.ts with IGroupListPagedSource interface
- Refactor postManager to use CachedManager and Sources pattern
- Refactor groupManager to use CachedManager and Sources pattern
- Clean up duplicate methods in groupService.ts
- Fix TypeScript errors and remove mock data

src/services/messageService.ts

@@ -27,6 +27,7 @@ import {
   saveConversationCache,
   saveConversationsWithRelatedCache,
   updateConversationCacheUnreadCount,
+  saveUsersCache,
 } from './database';
 
 /** 远端会话列表分页（offset / cursor）统一结果：拉取成功后均已执行本地缓存同步 */