Save button download the image without the frame border

Steps to reproduce:

- When I click the "Save" button
- My image is downloaded but the frame around the canvas isn't saved

Fix:

To fix it, the `.frame` and `.frame--inner` class from the [frame.svelte](https://github.com/cyrilf/unique/blob/main/src/lib/components/frame.svelte) file should be moved from the html/css into the canvas itself. The canvas area should be a bit larger and include the frame directly.

Read src/lib/components/frame.svelte and trace the Save button's canvas export path. Confirm how the .frame and .frame--inner styling currently surrounds the canvas, then verify that the downloaded image includes the frame and that the canvas area has the intended larger size.

Written by the indexing model from the issue text.