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/follow.go

@@ -0,0 +1,58 @@
+package handler
+
+import (
+	"net/http"
+	"strconv"
+
+	"github.com/freefire/jiang13-bbs/middleware"
+	"github.com/gin-gonic/gin"
+)
+
+// ToggleFollow 切换当前登录用户对目标用户的关注状态
+func (h *Handlers) ToggleFollow(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
+	}
+	claims := middleware.CurrentUser(c)
+	if claims == nil {
+		c.JSON(http.StatusUnauthorized, gin.H{"error": "未登录"})
+		return
+	}
+	following, err := h.Follow.Toggle(claims.ID, uint(id))
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"following": following})
+}
+
+// UserFollowing 获取用户关注的人列表（公开）
+func (h *Handlers) UserFollowing(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"))
+
+	users, total, err := h.Follow.ListFollowing(uint(id), page, size)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{
+		"users": nonNilSlice(users),
+		"total": total,
+		"page":  page,
+		"size":  size,
+	})
+}