fix(frontend): 修复 Markdown 代码块折叠空白，收起滚回块顶

避免客户端覆盖 SSR 高亮；空行撑高；收起后 scrollIntoView 到代码块头部。

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

frontend/components/markdown/renderMarkdown.tsx

@@ -17,6 +17,12 @@ export const prettyCodeOptions = {
   },
   keepBackground: false,
   defaultLang: "plaintext",
+  // 空行在 display:grid 下会塌缩成 0 高，补一个空格撑住行高
+  onVisitLine(element: { children: unknown[] }) {
+    if (element.children.length === 0) {
+      element.children = [{ type: "text", value: " " }];
+    }
+  },
 } as const;
 
 /** 服务端异步管线：GFM + Shiki 双主题高亮 → React 树 */