Add QoL features: preview theme, external resources, shortcuts, mobile layout

- Dark/light preview theme toggle with localStorage persistence and
  dark CSS injection in preview, export, and embed
- External CSS/JS resources modal with per-fiddle persistence in
  options column, injected as link/script tags
- Keyboard shortcuts cheat sheet modal (? button or ? key)
- Mobile-responsive CSS with breakpoints at 768px and 480px
  for both editor and browse pages

public/js/preferences.js

@@ -5,6 +5,7 @@ const DEFAULTS = {
   layout: 'default',
   keybindings: 'default',
   panelSizes: null,
+  previewTheme: 'light',
 };
 
 export function getPref(key) {