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/renderMarkdown.tsx

@@ -0,0 +1,45 @@
+import type { Components } from "react-markdown";
+import type { ReactElement } from "react";
+import { Fragment, jsx, jsxs } from "react/jsx-runtime";
+import { toJsxRuntime } from "hast-util-to-jsx-runtime";
+import { unified } from "unified";
+import remarkParse from "remark-parse";
+import remarkGfm from "remark-gfm";
+import remarkRehype from "remark-rehype";
+import rehypePrettyCode from "rehype-pretty-code";
+import type { Root } from "hast";
+import { mergeMarkdownComponents } from "./mergeComponents";
+
+export const prettyCodeOptions = {
+  theme: {
+    light: "github-light",
+    dark: "github-dark-dimmed",
+  },
+  keepBackground: false,
+  defaultLang: "plaintext",
+} as const;
+
+/** 服务端异步管线：GFM + Shiki 双主题高亮 → React 树 */
+export async function renderMarkdownToReact(
+  content: string,
+  overrides?: Components
+): Promise<ReactElement> {
+  const processor = unified()
+    .use(remarkParse)
+    .use(remarkGfm)
+    .use(remarkRehype)
+    .use(rehypePrettyCode, { ...prettyCodeOptions });
+
+  const mdast = processor.parse(content || "");
+  const hast = (await processor.run(mdast)) as Root;
+
+  return toJsxRuntime(hast, {
+    Fragment,
+    jsx,
+    jsxs,
+    components: mergeMarkdownComponents(overrides) as Record<
+      string,
+      React.ElementType
+    >,
+  }) as ReactElement;
+}