feat: 帖子收藏与用户关注

- 新增收藏功能：favorite 服务与 handler、FavoriteButton 组件
- 新增关注功能：follow 服务与 handler、FollowButton 组件
- 用户主页适配关注/粉丝与收藏展示
- flash 消息工具、广告与侧边栏组件微调

backend/handler/handlers.go

@@ -18,6 +18,8 @@ type Handlers struct {
 	Post         *service.PostService
 	Comment      *service.CommentService
 	Like         *service.LikeService
+	Favorite     *service.FavoriteService
+	Follow       *service.FollowService
 	Notification *service.NotificationService
 	OverviewSvc  *service.OverviewService
 	Checkin      *service.CheckinService