feat(renderer): MarkdownRenderer with mermaid code re-extraction from source

src/renderer/components/preview/MarkdownRenderer.tsx

@@ -0,0 +1,44 @@
+import { useEffect, useRef, useState } from 'react';
+import { renderMarkdown } from '@/lib/markdown';
+import { MermaidLazy } from './MermaidLazy';
+
+interface Props {
+  source: string;
+}
+
+const MERMAID_RE = /```mermaid\n([\s\S]*?)```/g;
+
+function extractMermaidCodes(source: string): string[] {
+  const codes: string[] = [];
+  let match: RegExpExecArray | null;
+  while ((match = MERMAID_RE.exec(source)) !== null) {
+    codes.push(match[1].trim());
+  }
+  return codes;
+}
+
+export function MarkdownRenderer({ source }: Props) {
+  const html = renderMarkdown(source);
+  const containerRef = useRef<HTMLDivElement>(null);
+  const [mermaidCodes, setMermaidCodes] = useState<string[]>([]);
+
+  // Re-derive mermaid codes from the original source. The renderMarkdown function
+  // replaces mermaid blocks with placeholders (data-mermaid-source="${idx}"), but
+  // it does not return the actual codes. Rather than thread them through the API,
+  // we re-extract from the source. Source is small (single buffer content) so this
+  // cost is negligible.
+  useEffect(() => {
+    setMermaidCodes(extractMermaidCodes(source));
+  }, [source]);
+
+  return (
+    <div className="prose prose-neutral dark:prose-invert max-w-none p-6" ref={containerRef}>
+      <div dangerouslySetInnerHTML={{ __html: html }} />
+      {mermaidCodes.map((code, i) => (
+        <div key={i} className="my-4">
+          <MermaidLazy code={code} />
+        </div>
+      ))}
+    </div>
+  );
+}