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/favorite.go

@@ -0,0 +1,69 @@
+package handler
+
+import (
+	"net/http"
+	"strconv"
+
+	"github.com/freefire/jiang13-bbs/middleware"
+	"github.com/gin-gonic/gin"
+)
+
+// ToggleFavorite 切换当前登录用户对帖子的收藏状态
+func (h *Handlers) ToggleFavorite(c *gin.Context) {
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "无效的帖子 ID"})
+		return
+	}
+	claims := middleware.CurrentUser(c)
+	if claims == nil {
+		c.JSON(http.StatusUnauthorized, gin.H{"error": "未登录"})
+		return
+	}
+	favorited, err := h.Favorite.Toggle(uint(id), claims.ID)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"favorited": favorited})
+}
+
+// UserFavorites 获取用户收藏的帖子列表（公开可见，仅 published）
+func (h *Handlers) UserFavorites(c *gin.Context) {
+	id, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "无效的用户 ID"})
+		return
+	}
+	if _, err := h.Auth.GetUserByID(uint(id)); err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "用户不存在"})
+		return
+	}
+
+	page, _ := strconv.Atoi(c.DefaultQuery("page", "1"))
+	size, _ := strconv.Atoi(c.DefaultQuery("size", "20"))
+
+	posts, total, err := h.Favorite.List(uint(id), page, size)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+	// 填充点赞状态（仅登录用户）
+	if claims := middleware.CurrentUser(c); claims != nil {
+		ids := make([]uint, 0, len(posts))
+		for _, p := range posts {
+			ids = append(ids, p.ID)
+		}
+		likedMap := h.Like.BatchHasLiked(ids, claims.ID)
+		for i := range posts {
+			posts[i].Liked = likedMap[posts[i].ID]
+		}
+	}
+
+	c.JSON(http.StatusOK, gin.H{
+		"posts": nonNilSlice(posts),
+		"total": total,
+		"page":  page,
+		"size":  size,
+	})
+}