perf: lazy-load ReactMarkdown and plugins for faster initial render

Move react-markdown, remark-gfm, remark-breaks, and rehype-highlight
imports from eager to dynamic via a LazyMarkdown wrapper component.
The ~336KB markdown bundle is now loaded on-demand after initial paint
instead of blocking the critical rendering path.

- Create LazyMarkdown component with Suspense fallback
- Pre-load plugins in parallel on module init
- Replace all ReactMarkdown usage in ChatMessage with LazyMarkdown

src/components/ChatMessage.tsx

@@ -1,11 +1,7 @@
 
 import { useState, useCallback, useRef, useEffect, useMemo, memo } from 'react';
 import { createPortal } from 'react-dom';
-import ReactMarkdown from 'react-markdown';
-import remarkGfm from 'remark-gfm';
-import remarkBreaks from 'remark-breaks';
-import rehypeHighlight from 'rehype-highlight';
-import { rehypeHighlightOptions } from '../lib/highlight';
+import { LazyMarkdown } from './LazyMarkdown';
 import type { ChatMessage as ChatMessageType, MessageBlock } from '../types';
 import { useTheme } from '../hooks/useTheme';
 import { ThinkingBlock } from './ThinkingBlock';
@@ -167,17 +163,13 @@ function MarkdownLink(props: React.AnchorHTMLAttributes<HTMLAnchorElement>) {
 }
 
 const markdownComponents = { pre: CodeBlock, img: MarkdownImage, a: MarkdownLink };
-// Hoisted to module scope to avoid creating new array references each render
-import type { PluggableList } from 'unified';
-const remarkPlugins: PluggableList = [remarkGfm, remarkBreaks];
-const rehypePlugins: PluggableList = [[rehypeHighlight, rehypeHighlightOptions]];
 
 function renderTextBlocks(blocks: MessageBlock[]) {
   return getTextBlocks(blocks).map((block, i) => (
     <div key={`text-${i}`} className="markdown-body">
-      <ReactMarkdown remarkPlugins={remarkPlugins} rehypePlugins={rehypePlugins} components={markdownComponents}>
+      <LazyMarkdown components={markdownComponents}>
         {autoFormatText((block as Extract<MessageBlock, { type: 'text' }>).text)}
-      </ReactMarkdown>
+      </LazyMarkdown>
     </div>
   ));
 }
@@ -480,9 +472,9 @@ export const ChatMessageComponent = memo(function ChatMessageComponent({ message
           {/* User-visible text */}
           {message.blocks.length > 0 ? renderTextBlocks(message.blocks) : (
             <div className="markdown-body">
-              <ReactMarkdown remarkPlugins={remarkPlugins} rehypePlugins={rehypePlugins} components={markdownComponents}>
+              <LazyMarkdown components={markdownComponents}>
                 {autoFormatText(message.content)}
-              </ReactMarkdown>
+              </LazyMarkdown>
             </div>
           )}
 

src/components/LazyMarkdown.tsx

@@ -0,0 +1,50 @@
+import { lazy, Suspense } from 'react';
+import type { Components } from 'react-markdown';
+import type { PluggableList } from 'unified';
+
+const ReactMarkdown = lazy(() => import('react-markdown'));
+
+// Pre-load plugins so they're ready when ReactMarkdown renders
+let _remarkPlugins: PluggableList | null = null;
+let _rehypePlugins: PluggableList | null = null;
+let _pluginsReady = false;
+
+const pluginsPromise = Promise.all([
+  import('remark-gfm').then(m => m.default),
+  import('remark-breaks').then(m => m.default),
+  import('rehype-highlight').then(m => m.default),
+  import('../lib/highlight').then(m => m.rehypeHighlightOptions),
+]).then(([remarkGfm, remarkBreaks, rehypeHighlight, rehypeHighlightOptions]) => {
+  _remarkPlugins = [remarkGfm, remarkBreaks];
+  _rehypePlugins = [[rehypeHighlight, rehypeHighlightOptions]];
+  _pluginsReady = true;
+});
+
+// Trigger plugin loading immediately
+void pluginsPromise;
+
+interface LazyMarkdownProps {
+  children: string;
+  remarkPlugins?: PluggableList;
+  rehypePlugins?: PluggableList;
+  components?: Components;
+}
+
+function MarkdownInner({ children, remarkPlugins, rehypePlugins, components }: LazyMarkdownProps) {
+  const remark = remarkPlugins ?? (_pluginsReady ? _remarkPlugins! : []);
+  const rehype = rehypePlugins ?? (_pluginsReady ? _rehypePlugins! : []);
+
+  return (
+    <ReactMarkdown remarkPlugins={remark} rehypePlugins={rehype} components={components}>
+      {children}
+    </ReactMarkdown>
+  );
+}
+
+export function LazyMarkdown(props: LazyMarkdownProps) {
+  return (
+    <Suspense fallback={<span className="opacity-50">{props.children.slice(0, 200)}</span>}>
+      <MarkdownInner {...props} />
+    </Suspense>
+  );
+}