feat(ui): render markdown in peek pane

- Add marked.js for full markdown rendering
- Stream peek body renders as markdown
- Card peek non-code content renders as markdown
- Code/snippet cards keep escaped pre/code display
- Styled: headers, lists, blockquotes, inline code, code blocks, links, hr
- Graceful fallback to escHtml if marked fails to load

web/index.html

@@ -82,6 +82,7 @@
         </div>
     </div>
 
+    <script src="https://cdn.jsdelivr.net/npm/marked@15/marked.min.js"></script>
     <script src="/app.js"></script>
 </body>
 </html>