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/console-panel.js

@@ -1,6 +1,10 @@
+import { registerClearHandler } from './devtools.js';
+
 const output = () => document.getElementById('console-output');
 
 export function initConsole() {
+  registerClearHandler('console', clearConsole);
+
   window.addEventListener('message', (e) => {
     if (!e.data || e.data.type !== 'console') return;
     if (e.data.method === 'clear') {
@@ -9,8 +13,6 @@ export function initConsole() {
     }
     appendLine(e.data.method, (e.data.args || []).join(' '));
   });
-
-  document.getElementById('btn-clear-console').addEventListener('click', clearConsole);
 }
 
 function appendLine(method, text) {