Better UI for random seed on onsite generation

Currently the generator makes a random seed for the default "fixed" seed everytime the user switches to fixed from random seed. This isn't very intuitive: The natural expectation would be that the "fixed" seed field stays the same, but is ignored if "random" is selected. This is annoying if one wants to normally operate with a fixed seed, but sometimes try a prompt with a random seed to see if any issues are seed specific.

I think it would be better if instead the "fixed" seed's current text remained the same, just ignored when "Random" was active, and there was a small button (a ⚂ about the size as the clear seed button) that rerolled the current fixed seed.