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>

frontend/components/admin/AdminShell.tsx

@@ -21,6 +21,7 @@ import {
   User as UserIcon,
   Settings,
   ChevronDown,
+  FileText,
 } from "lucide-react";
 import type { User } from "@/lib/api";
 import { apiAdminPendingCounts, apiLogout } from "@/lib/api";
@@ -83,6 +84,12 @@ const NAV_GROUPS: NavGroup[] = [
         icon: Megaphone,
         visible: (u) => isAdminOrAbove(u.role),
       },
+      {
+        href: "/admin/pages",
+        label: "单页",
+        icon: FileText,
+        visible: (u) => isAdminOrAbove(u.role),
+      },
     ],
   },
   {