feat(demo): add Mermaid.js CDN for diagram rendering in demo page

- Load mermaid@10 from jsdelivr CDN (library remains zero-dependency)
- Auto-render on afterRender event + initial timeout
- mermaid.run() targets .me-mermaid .mermaid containers

site/demo.html

@@ -97,6 +97,7 @@ body{font-family:-apple-system,BlinkMacSystemFont,"Segoe UI","PingFang SC","Micr
   <a href="https://git.metona.cn/MetonaTeam/MetonaEditor" target="_blank" rel="noopener">源码仓库</a> · MIT
 </footer>
 
+<script src="https://cdn.jsdelivr.net/npm/mermaid@10/dist/mermaid.min.js"></script>
 <script src="../dist/metona-editor.js"></script>
 <script>
 (function(){
@@ -262,6 +263,15 @@ editor.on('fileOpened',function(d){editor.toast('已打开: '+d.name,{type:'succ
 editor.on('fileSaved',function(){editor.toast('文件已保存',{type:'success'})});
 editor.on('linkClick',function(d){editor.toast('链接点击: '+d.href,{type:'info',duration:1500})});
 
+// Mermaid 图表渲染（v0.1.14）
+function renderMermaid(){
+  if(typeof mermaid==='undefined')return;
+  try{mermaid.run({querySelector:'.me-mermaid .mermaid'})}catch(e){}
+}
+mermaid.initialize({startOnLoad:false,theme:'default'});
+editor.on('afterRender',renderMermaid);
+setTimeout(renderMermaid,300);
+
 })();
 </script>
 </body>