Add a better looking frame for mobile and web previews 

Something similar to this:

<img width="372" alt="Screenshot 2023-12-10 at 10 24 26 AM" src="https://github.com/abi/screenshot-to-code/assets/23818/3aa4ad4b-0e91-44e9-8dd5-be9914782dbd">
<img width="876" alt="Screenshot 2023-12-10 at 10 24 35 AM" src="https://github.com/abi/screenshot-to-code/assets/23818/1306a725-6e23-4f17-baad-3ec91bbe205d">
