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/app/admin/pages/new/page.tsx

@@ -0,0 +1,21 @@
+import type { Metadata } from "next";
+import { cookies } from "next/headers";
+import { authCookieHeader } from "@/lib/cookies";
+import { getMeCached } from "@/lib/serverData";
+import { isAdminOrAbove } from "@/lib/roles";
+import Forbidden from "../../Forbidden";
+import PageComposeClient from "../PageComposeClient";
+
+export const metadata: Metadata = {
+  title: "新建单页",
+};
+
+export default async function AdminPageNewPage() {
+  const cookie = authCookieHeader(await cookies());
+  const me = await getMeCached(cookie || undefined);
+  if (!me.user || !isAdminOrAbove(me.user.role)) {
+    return <Forbidden text="单页管理仅管理员及以上角色可操作。" />;
+  }
+
+  return <PageComposeClient mode="create" />;
+}