feat: add Shadcn components

[martakam]

Adds multiple Shadcn components that we'll likely use.
If any of them won't be used, we can always remove them later on.

Note on the Shadcn CLI usage:
When adding new components using the CLI, the file names will be in kebab-case.tsx, for example alert-dialog.tsx.
We use PascalCase.tsx, so that would be AlertDialog.tsx instead.

If you'll import new components, please make sure to change the casing of the files in order to match.
A note about this will be added to CONTRIBUTING.md once #15 is merged.

[johahor]

Looks good in general, but one thing i notices was in the index.css file.

There are two @theme blocks, and the second one is the one that wins. One place i notices a problem was in the "Previous" and "Next" buttons. I'll provide an image as well.

This is not the biggest issue, and since @hestro is planning to convert out components to Shadcn, I'm fine with approving this, and getting it merged, so that the important backend/database work can build on top of this to avoid possible conflicts. #22 moves all frontent files to /frontend, so that will probably get conflicts. Better to fix now, than later. So my suggestion is merge this as is, and update #22 with the updated main, and then continue the backend work. Then @hestro can work independently on the frontend while others work on the backend without worrying about conflicts.

[martakam]

The index.css file and the general look should be improved now. When components are changed to use Shadcn, the usage of colors and borders should also become more standardized. For example, all buttons will be using the component Button from Shadcn with predefined styling for different variants (default, secondary, destructive, etc).

[johahor]

Looks good, approving. Delivery 2 isn't that centered around perfect UI, so make the UI spotless is something we can deal with later. Now we have the tools to do it, so good work.