feat: markdown rendering for response viewer

Add marked.js (39KB) for rich text display in the response viewer.
Renders headings, code blocks, lists, tables, blockquotes, and
inline formatting with dark theme styling.

Falls back to escaped plain text if marked.js fails to load.

src/web/public/index.html

@@ -26,6 +26,7 @@
   <!-- WebGL addon lazy-loaded by app.js on desktop only (skipped on mobile, saving 244KB) -->
   <script defer src="vendor/xterm-addon-unicode11.min.js"></script>
   <script defer src="vendor/xterm-zerolag-input.js"></script>
+  <script defer src="vendor/marked.min.js"></script>
   <!-- Synchronous mobile detection — runs before first paint to prevent panel flash -->
   <script>if(window.innerWidth<768||(('ontouchstart' in window||navigator.maxTouchPoints>0)&&window.innerWidth<1024))document.documentElement.classList.add('mobile-init');</script>
   <!-- Inline critical CSS for instant skeleton paint (before styles.css loads) -->