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/components/markdown/MdLink.tsx

@@ -0,0 +1,38 @@
+"use client";
+
+import type { AnchorHTMLAttributes } from "react";
+
+type MdLinkProps = AnchorHTMLAttributes<HTMLAnchorElement>;
+
+function isExternalHref(href?: string): boolean {
+  if (!href) return false;
+  if (href.startsWith("/") || href.startsWith("#") || href.startsWith("?")) {
+    return false;
+  }
+  if (href.startsWith("mailto:") || href.startsWith("tel:")) return false;
+  try {
+    if (typeof window !== "undefined") {
+      const url = new URL(href, window.location.origin);
+      return url.origin !== window.location.origin;
+    }
+  } catch {
+    /* 相对或非法 URL 当站内 */
+  }
+  return /^https?:\/\//i.test(href);
+}
+
+/** Markdown 链接：外链新开页 */
+export default function MdLink({ href, children, ...rest }: MdLinkProps) {
+  const external = isExternalHref(href);
+  return (
+    <a
+      {...rest}
+      href={href}
+      {...(external
+        ? { target: "_blank", rel: "noopener noreferrer" }
+        : {})}
+    >
+      {children}
+    </a>
+  );
+}