Make IconButton on compound-web fixed sizes

# Platform

P-Web

- ...

# Package version

- ...

# Description

IconButton calculates the size of the icon / padding from the total size given as the icon being 0.75 the size of the button, but that doesn't really work because of rounding because a 28px button ends up with a 21px icon where it should be 20px. In the EW room list icons, we actually override the padding to work around this.

We should just allow a fixed set of sizes, each with fixed icon / padding sizes. It should be (from aaron):

Global secondary buttons = 28px with 20px icon
Global primary buttons = 36px with 24px icon
Quick action buttons = 48px with 24px icon

Start by locating IconButton in compound-web and inspect how its current total-size calculation derives the icon and padding. Implement the three fixed size combinations described in the issue, then verify that secondary, primary, and quick-action buttons use the stated dimensions.

Written by the indexing model from the issue text.