Quick-and-dirty corner roundness toggle

Created by: ShittyKopper

To keep things manageable I merged a lot of one off values into just a handful of common sizes, so some parts of the UI will look different than upstream even with the "Misskey" rounding mode.

A lot of find and replace were involved, so I definitely need another set of eyes to look over this and say what parts should be (un)rounded, marking as draft for now.