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/model/db.go

@@ -57,7 +57,7 @@ func InitDB(dsn string) error {
 
 	if err := db.AutoMigrate(
 		&TemporaryUpload{}, &ModuleConfig{}, &SettingsAudit{}, &ActionCounter{}, &MailTask{}, &EmailChallenge{}, &StoredObject{},
-		&User{}, &Board{}, &Post{}, &PostEditHistory{}, &Comment{}, &CommentEditHistory{}, &RefreshToken{}, &Like{}, &Notification{}, &Checkin{},
+		&User{}, &Board{}, &Post{}, &PostEditHistory{}, &Comment{}, &CommentEditHistory{}, &RefreshToken{}, &Like{}, &Favorite{}, &Follow{}, &Notification{}, &Checkin{},
 		&Announcement{}, &SitePage{}, &SiteSetting{}, &SiteDailyStats{}, &SiteDailyVisitor{}, &VisitEvent{}, &Attachment{}, &UserBoard{}, &LoginLog{},
 		&ChatRoom{}, &ChatRoomMember{}, &ChatMessage{},
 		&PointLedger{}, &PostContentUnlock{}, &PostAttachment{}, &PostAttachmentUnlock{},