Build a single-file HTML app (index.html) that is a simple markdown note-taking app:

- A textarea where you type markdown
- A live preview pane next to it that renders the markdown as HTML
- Use a simple markdown parser (you can write a basic one inline or use a CDN script)
- Add a "Save" button that downloads the markdown as a .md file
- Make it look clean with some CSS — dark theme, monospace font for the editor
- Should work offline, no backend needed

Create index.html in this directory.
