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/preferences.js

@@ -6,6 +6,8 @@ const DEFAULTS = {
   keybindings: 'default',
   panelSizes: null,
   previewTheme: 'light',
+  previewDevice: 'desktop',
+  editorTheme: 'vs-dark',
 };
 
 export function getPref(key) {