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This is nifty. Is there any similar tool that presents the UI as a web page, rather than a GTK gui?

GTK actually seems to be capable of this for many years, with its "Broadway" GDK backend:

You can run broadway inside a container as well. Yad could make an interesting entrypoint for it

NodeJS, PHP, Python, Perl?

Write the WebUI in a separate HTML page, then start a server and listen for requests.