feat: rich markdown renderer with @mention badges and autocomplete

- Add MarkdownRenderer with syntax-highlighted code blocks (prism-react-renderer),
  GFM tables, and Discord-style theming
- Add MentionBadge that resolves user IDs to display names with role colors
- Add MentionPopover autocomplete triggered by @ in MessageInput
- Fix mention:// URL sanitization — allowlist the scheme in urlTransform so
  react-markdown v9 passes it through to the component override
- Highlight messages that mention the current user (amber border)
- Bump rate limit from 60 to 200 req/min

packages/server/src/index.ts

@@ -36,7 +36,7 @@ async function main(): Promise<void> {
   });
 
   await app.register(rateLimit, {
-    max: 60,
+    max: 200,
     timeWindow: '1 minute',
     keyGenerator: (request) => (request as any).userId || request.ip,
   });