公开页补齐自引用 Canonical 与按类型的结构化数据，图片补上说明文字。

后台设置和内容编辑改成可导航布局；部署用访客 origin 生成 Canonical，避免落到容器地址。

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

frontend/app/announcements/page.tsx

@@ -14,16 +14,23 @@ import {
   SiteDocPageNav,
   SiteDocBreadcrumb,
 } from "@/components/site-doc";
-
-export const metadata: Metadata = {
-  title: "站点公告",
-  description: "站点公告与通知列表",
-};
+import { requestCanonical } from "@/lib/canonicalMeta";
+import { canonicalPage } from "@/lib/siteUrl";
 
 interface PageProps {
   searchParams: Promise<{ page?: string }>;
 }
 
+export async function generateMetadata({ searchParams }: PageProps): Promise<Metadata> {
+  const sp = await searchParams;
+  const page = canonicalPage(sp.page);
+  return {
+    title: "站点公告",
+    description: "站点公告与通知列表",
+    ...(await requestCanonical("/announcements", page ? { page } : undefined)),
+  };
+}
+
 /** 粗略去掉 Markdown，取摘要 */
 function plainSummary(md: string, max = 120): string {
   const text = md