feat: add PAT support with custom Git server compatibility

[cevatkerim]

• Add optional PAT field with toggle checkbox in UI
• Support custom Git server API endpoints (api/v1/repos)
• Add comprehensive test coverage for PAT authentication
• Update repository existence check to handle both GitHub and custom Git servers

[cyclotruc]

Very clean minimal implementaiton, I like that the tokens never leave RAM as it reduces the security risk of handling user tokens
Thank you very much

[cyclotruc]

My bad: I missed those 4 lines while merging and I can't seem to be able to push those changes on your PR

[cyclotruc]

@cevatkerim I was quite reluctant at first to introduce PAT in gitingest but this first implementation seems very reasonable
Thank you very much

I would just like to improve the UX a bit before merging, if you can give me access to your branch I will glaadly implement those changes myself:

• The ingest button should remain as it was before on the same line as the search bar in desktop mode
• Make the checkbox and new textinput look more like the include/exclude input (mostly smaller)

[cyclotruc]

Update:

Since gitingest is focused on butter smooth experience rather than having many features, I'm considering not pushing this first step (PAT) in production but instead wait until we have proper OAuth going on with Github login

I'm going to keep this branch as a reference implementation for PAT which I had not much experience with, so your work is still very helpful!