Add Pull Request "viewed" button

[marcoieni]

When reviewing PRs, I review file by file, using the viewed button.
I miss this button from the T3code PR review panel:

[michaelst]

I would love this in the diff view as well

[marcoieni]

I have a working branch: https://github.com/marcoieni/t3code/tree/feat/github-pr-viewed
Done with astra in 3 prompts, I haven't reviewed the code yet. But I tested it and it works 👍

[marcoieni]

raised a PR #9969

[marcoieni]

My PR was closed in favor of #7721