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/favorite.go

@@ -0,0 +1,104 @@
+package service
+
+import (
+	"errors"
+
+	"github.com/freefire/jiang13-bbs/model"
+	"gorm.io/gorm"
+)
+
+// FavoriteService 收藏服务：收藏公开可见，公开列表只出 published 帖
+type FavoriteService struct {
+	db    *gorm.DB
+	posts *PostService
+}
+
+func NewFavoriteService(db *gorm.DB) *FavoriteService {
+	return &FavoriteService{db: db}
+}
+
+// WithPosts 注入帖子服务（列表复用 PostListItem 组装与最后回复填充）
+func (s *FavoriteService) WithPosts(p *PostService) *FavoriteService {
+	s.posts = p
+	return s
+}
+
+// Toggle 切换收藏状态：已收藏则取消，未收藏则收藏。返回 (favorited, error)
+func (s *FavoriteService) Toggle(postID, userID uint) (bool, error) {
+	var post model.Post
+	if err := s.db.First(&post, postID).Error; err != nil {
+		return false, errors.New("帖子不存在")
+	}
+
+	var fav model.Favorite
+	err := s.db.Where("post_id = ? AND user_id = ?", postID, userID).First(&fav).Error
+	if err == nil {
+		if err := s.db.Delete(&fav).Error; err != nil {
+			return true, err
+		}
+		return false, nil
+	}
+	if !errors.Is(err, gorm.ErrRecordNotFound) {
+		return false, err
+	}
+	if err := s.db.Create(&model.Favorite{PostID: postID, UserID: userID}).Error; err != nil {
+		return false, err
+	}
+	return true, nil
+}
+
+// HasFavorited 判断用户是否已收藏某帖子
+func (s *FavoriteService) HasFavorited(postID, userID uint) bool {
+	var count int64
+	s.db.Model(&model.Favorite{}).Where("post_id = ? AND user_id = ?", postID, userID).Count(&count)
+	return count > 0
+}
+
+// BatchHasFavorited 批量查询用户对多个帖子的收藏状态，返回 postID -> bool
+func (s *FavoriteService) BatchHasFavorited(postIDs []uint, userID uint) map[uint]bool {
+	result := make(map[uint]bool)
+	if len(postIDs) == 0 || userID == 0 {
+		return result
+	}
+	var favs []model.Favorite
+	s.db.Where("post_id IN ? AND user_id = ?", postIDs, userID).Find(&favs)
+	for _, f := range favs {
+		result[f.PostID] = true
+	}
+	return result
+}
+
+// List 用户收藏的帖子列表（公开可见，仅 published；按收藏时间倒序）
+func (s *FavoriteService) List(userID uint, page, size int) ([]PostListItem, int64, error) {
+	if page < 1 {
+		page = 1
+	}
+	if size < 1 || size > 50 {
+		size = 20
+	}
+	query := s.db.Model(&model.Post{}).
+		Joins("JOIN favorites ON favorites.post_id = posts.id").
+		Where("favorites.user_id = ? AND posts.status = ?", userID, model.ContentStatusPublished)
+	var total int64
+	if err := query.Count(&total).Error; err != nil {
+		return nil, 0, err
+	}
+	var posts []model.Post
+	offset := (page - 1) * size
+	if err := query.Select("posts.*").Order("favorites.created_at DESC").Offset(offset).Limit(size).
+		Preload("Board").Preload("User").Preload("User.Badges.Badge").Find(&posts).Error; err != nil {
+		return nil, 0, err
+	}
+	items := toPostListItems(posts)
+	if s.posts != nil {
+		s.posts.fillLastReply(items)
+	}
+	return items, total, nil
+}
+
+// CountByUser 用户收藏数量
+func (s *FavoriteService) CountByUser(userID uint) int64 {
+	var count int64
+	s.db.Model(&model.Favorite{}).Where("user_id = ?", userID).Count(&count)
+	return count
+}