fix: show sandbox flowcharts on Win7 Chrome and repair common Mermaid syntax

Disable HTML labels so nodes use native SVG text, and rewrite bare edge labels plus invalid class statements before render.

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

frontend-web/src/components/workspace/messages/markdown-content.tsx

@@ -143,6 +143,7 @@ export function MarkdownContent({
         className={className}
         remarkPlugins={remarkPlugins}
         rehypePlugins={rehypePlugins}
+        mermaidConfig={streamdownPlugins.mermaidConfig}
         components={components}
       >
         {content}