feat:connect frontend with actual backend endpoints + delete:unused f…

[kazuhidelee]

…iles from previous frontend version

Description

This is a draft for now

• Enabling frontend to use actual data from backend api to generate the visualizer
• Deleted all the unused files and states from previous version of the visualizer
  For testing:
• start frontend in the frontend folder using: npm run dev
• start backend in the backend folder using: go run cmd/server/main.go
• use github.com/gittuf/gittuf to test the visualizer (can also use the local path to your local gittuf repo) or any other repo that contains gittuf metadata
  (for now settings page is frontend only, and the graph is using both root.json +targets.json)

AI Usage

• I did not use generative AI at all in making the content of this pull
  request.
• I did use generative AI in some form in making the content of this
  pull request. I have described my use of AI below.

Contributor Checklist

• I have manually reviewed all content submitted to gittuf in this pull
  request.
• I fully understand the content I am submitting.
• The changes introduced are documented and have tests included if
  applicable.
• My changes do not infringe on copyright/trademarks/etc.
• All commits in this pull request include a DCO
  Signoff.
• By submitting this pull request, I agree to follow the gittuf Code of
  Conduct.

[patzielinski]

Please also clean up the merge commits on the branch, thanks!