feat: 站点单页、时间线导入与行级 BBCode 短代码

首页右栏接入单页入口与公告置顶/全部页；Markdown 编辑器支持可视化时间线与 Git 导入；隐藏块改为 [hide]/[timeline] 命名闭合，并修复单页/公告 GORM 更新只改 updated_at 的问题。

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

backend/handler/overview.go

@@ -29,12 +29,14 @@ func (h *Handlers) Overview(c *gin.Context) {
 		}
 	}
 	c.JSON(http.StatusOK, gin.H{
-		"stats":         data.Stats,
-		"hot":           data.Hot,
-		"active_users":  data.ActiveUsers,
-		"boards":        data.Boards,
-		"announcements": data.Announcements,
-		"new_users":     data.NewUsers,
-		"checkin":       data.Checkin,
+		"stats":               data.Stats,
+		"hot":                 data.Hot,
+		"active_users":        data.ActiveUsers,
+		"boards":              data.Boards,
+		"announcements":       data.Announcements,
+		"announcements_total": data.AnnouncementsTotal,
+		"sidebar_pages":       data.SidebarPages,
+		"new_users":           data.NewUsers,
+		"checkin":             data.Checkin,
 	})
 }