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/preferences.js

@@ -0,0 +1,22 @@
+const PREFIX = 'fiddle_';
+
+const DEFAULTS = {
+  autoRun: true,
+  layout: 'default',
+  keybindings: 'default',
+  panelSizes: null,
+};
+
+export function getPref(key) {
+  const raw = localStorage.getItem(PREFIX + key);
+  if (raw === null) return DEFAULTS[key] ?? null;
+  try { return JSON.parse(raw); } catch { return raw; }
+}
+
+export function setPref(key, value) {
+  localStorage.setItem(PREFIX + key, JSON.stringify(value));
+}
+
+export function removePref(key) {
+  localStorage.removeItem(PREFIX + key);
+}