feat(frontend): Markdown 支持 GFM、代码高亮与图片灯箱

统一 SSR/客户端渲染路径，评论与预览共用增强组件。

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

frontend/app/announcement/[id]/page.tsx

@@ -1,10 +1,10 @@
 import type { Metadata } from "next";
 import Link from "next/link";
 import { notFound } from "next/navigation";
-import ReactMarkdown from "react-markdown";
 import { ChevronRight, Megaphone } from "lucide-react";
 import { fetchAnnouncementDetail } from "@/lib/api";
 import { formatDate } from "@/lib/format";
+import MarkdownBody from "@/components/MarkdownBody";
 
 interface PageProps {
   params: Promise<{ id: string }>;
@@ -77,9 +77,7 @@ export default async function AnnouncementPage({ params }: PageProps) {
           className="mt-5 pt-5"
           style={{ borderTop: "1px solid var(--line)" }}
         >
-          <div className="prose-content">
-            <ReactMarkdown>{a.content}</ReactMarkdown>
-          </div>
+          <MarkdownBody content={a.content} />
         </div>
       </article>