Nice, I'm proud I managed to nerd-snipe you :-) Thanks for taking the time.

Seriously, though, I think this solves a nicely framed simpler problem. I was thinking about a more general tool, but that's genuinely hard (you'll need heavy CV algorithms or a special ML model to detect what is background what what isn't).

To be honest, what you built here is probably sufficient anyway, because the models are better at obeying "create a white background" or "create a 0xffffff background" than "transparent", so this tool can post-process to what's needed.

When asked for "transparent", I've had a model generate a fake checkerboard pattern of gray colors to imitate how viewers render transparent areas :-) For this kind of nonsense, the transparent-png tool wouldn't do!

The best thing to say to an image model IME is "use a colour/chroma key background" and then it gives you green or blue or whatever depending on colours used in the image.