Is there a Web UI equivalent for tools like this, where one can spin up a web server pointing to a git repo on the server and do commit, diff, merging, graph visualisation etc. through a web interface?
Not exactly what you are looking for, but github.dev provides much of this for repos hosted at github. Press . while accessing the repository at github.com to jump right into github.dev (this appears to require me being logged in so it's possible I configured this somehow).