- Add isHtmlFile() helper to detect .html/.htm files
- Add htmlViewMode state for preview/edit toggle
- Add localStorage persistence for HTML view mode preference
- Show PreviewToggleButton for HTML files
- Render HTML in sandboxed iframe with srcdoc
- Inject base tag for relative CSS/JS/image paths to work
- Sandbox permissions: allow-scripts, same-origin, forms
- Reset view mode when switching files