Add responsive preview, editor themes, template gallery, devtools, and autocomplete

- Device breakpoint toggles (mobile 375px / tablet 768px / desktop 100%)
- Editor theme selector with 6 themes (VS Dark/Light, High Contrast, Monokai, Dracula, GitHub Dark)
- Starter template gallery with 8 pre-built templates (Todo, API Fetch, CSS Animation, etc.)
- Code autocomplete with DOM/React type definitions and snippet completions
- Devtools panels: console, network, elements, performance
- Code formatter (Prettier), diff view, and linter integration

public/js/embed.js

@@ -76,7 +76,7 @@ async function init() {
     }
 
     const tailwindScript = opts.tailwind
-      ? `<script src="https://cdn.tailwindcss.com"><\/script>\n`
+      ? `<script>var _tw=console.warn;console.warn=function(){if(typeof arguments[0]==='string'&&arguments[0].indexOf('cdn.tailwindcss.com')!==-1)return;_tw.apply(console,arguments)}<\/script>\n<script src="https://cdn.tailwindcss.com"><\/script>\n<script>console.warn=_tw<\/script>\n`
       : '';
 
     // Dark preview theme — from fiddle options or URL param