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/service/follow.go

@@ -0,0 +1,104 @@
+package service
+
+import (
+	"errors"
+
+	"github.com/freefire/jiang13-bbs/model"
+	"gorm.io/gorm"
+)
+
+// FollowService 关注服务（仅用户；关注时给对方发通知）
+type FollowService struct {
+	db   *gorm.DB
+	notif *NotificationService
+}
+
+func NewFollowService(db *gorm.DB) *FollowService {
+	return &FollowService{db: db}
+}
+
+// WithNotification 注入通知服务（关注成功时落库一条 follow 通知）
+func (s *FollowService) WithNotification(n *NotificationService) *FollowService {
+	s.notif = n
+	return s
+}
+
+// Toggle 切换关注状态：已关注则取关，未关注则关注。返回 (following, error)
+func (s *FollowService) Toggle(followerID, targetID uint) (bool, error) {
+	if followerID == targetID {
+		return false, errors.New("不能关注自己")
+	}
+	var target model.User
+	if err := s.db.First(&target, targetID).Error; err != nil {
+		return false, errors.New("用户不存在")
+	}
+	if target.Banned {
+		return false, errors.New("该用户已被封禁")
+	}
+
+	var follow model.Follow
+	err := s.db.Where("follower_id = ? AND following_id = ?", followerID, targetID).First(&follow).Error
+	if err == nil {
+		if err := s.db.Delete(&follow).Error; err != nil {
+			return true, err
+		}
+		return false, nil
+	}
+	if !errors.Is(err, gorm.ErrRecordNotFound) {
+		return false, err
+	}
+	if err := s.db.Create(&model.Follow{FollowerID: followerID, FollowingID: targetID}).Error; err != nil {
+		return false, err
+	}
+	if s.notif != nil {
+		s.notif.Create(targetID, followerID, model.NotificationTypeFollow, 0, 0, "")
+	}
+	return true, nil
+}
+
+// IsFollowing 判断 followerID 是否已关注 targetID
+func (s *FollowService) IsFollowing(followerID, targetID uint) bool {
+	if followerID == 0 || targetID == 0 || followerID == targetID {
+		return false
+	}
+	var count int64
+	s.db.Model(&model.Follow{}).Where("follower_id = ? AND following_id = ?", followerID, targetID).Count(&count)
+	return count > 0
+}
+
+// ListFollowing 用户关注的人列表（公开；按关注时间倒序）
+func (s *FollowService) ListFollowing(userID uint, page, size int) ([]model.User, int64, error) {
+	if page < 1 {
+		page = 1
+	}
+	if size < 1 || size > 50 {
+		size = 20
+	}
+	query := s.db.Model(&model.User{}).
+		Joins("JOIN follows ON follows.following_id = users.id").
+		Where("follows.follower_id = ?", userID)
+	var total int64
+	if err := query.Count(&total).Error; err != nil {
+		return nil, 0, err
+	}
+	var users []model.User
+	offset := (page - 1) * size
+	if err := query.Select("users.*").Order("follows.created_at DESC").Offset(offset).Limit(size).Find(&users).Error; err != nil {
+		return nil, 0, err
+	}
+	return users, total, nil
+}
+
+// CountFollowing 关注数量
+func (s *FollowService) CountFollowing(userID uint) int64 {
+	var count int64
+	s.db.Model(&model.Follow{}).Where("follower_id = ?", userID).Count(&count)
+	return count
+}
+
+// CountFollowers 粉丝数量
+func (s *FollowService) CountFollowers(userID uint) int64 {
+	var count int64
+	s.db.Model(&model.Follow{}).Where("following_id = ?", userID).Count(&count)
+	return count
+}