Add editor experience features: auto-run, layouts, resizable panels, Emmet, vim/emacs

- Auto-run toggle with localStorage persistence (toolbar checkbox)
- Layout selector: default, top/bottom, editor-only, preview-only
- Resizable panels via drag dividers with double-click reset
- Emmet abbreviation expansion for HTML, CSS, and JSX
- Vim and Emacs keybinding modes with lazy-loaded CDN libraries
- Shared preferences module for localStorage management
- Editor hooks for tab switch and mode change callbacks

public/js/emmet.js

@@ -0,0 +1,29 @@
+let loaded = false;
+
+function loadScript(src) {
+  return new Promise((resolve, reject) => {
+    if (document.querySelector(`script[src="${src}"]`)) { resolve(); return; }
+    const savedDefine = window.define;
+    window.define = undefined;
+    const s = document.createElement('script');
+    s.src = src;
+    s.onload = () => { window.define = savedDefine; resolve(); };
+    s.onerror = () => { window.define = savedDefine; reject(new Error(`Failed to load ${src}`)); };
+    document.head.appendChild(s);
+  });
+}
+
+export async function initEmmet() {
+  if (loaded) return;
+  try {
+    await loadScript('https://unpkg.com/emmet-monaco-es/dist/emmet-monaco.min.js');
+    if (window.emmetMonaco) {
+      emmetMonaco.emmetHTML(monaco);
+      emmetMonaco.emmetCSS(monaco);
+      emmetMonaco.emmetJSX(monaco);
+      loaded = true;
+    }
+  } catch (e) {
+    console.warn('Emmet load failed:', e.message);
+  }
+}