fix: 空评论列表 500，并完善撤回审计与角色说明

空切片 JSON 统一为数组；撤回保留正文供站长阅读，成员侧居中提示与重新编辑；头像支持拖放；角色弹窗补全职责说明并汇入封禁。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/handler/announcement.go

@@ -19,7 +19,7 @@ func (h *Handlers) AnnouncementsList(c *gin.Context) {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "获取公告失败"})
 		return
 	}
-	c.JSON(http.StatusOK, gin.H{"announcements": list})
+	c.JSON(http.StatusOK, gin.H{"announcements": nonNilSlice(list)})
 }
 
 // AnnouncementDetail 已发布公告详情
@@ -50,7 +50,7 @@ func (h *Handlers) AdminListAnnouncements(c *gin.Context) {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "获取公告失败"})
 		return
 	}
-	c.JSON(http.StatusOK, gin.H{"announcements": list})
+	c.JSON(http.StatusOK, gin.H{"announcements": nonNilSlice(list)})
 }
 
 // AdminCreateAnnouncement 新建公告