Replaces the font-size-adjust correction with explicit control, mirroring
the mobile Appearance sliders: interface, prompt, code, and terminal each
get a size in CSS pixels alongside their family.

The x-height correction could never satisfy every face - matching exactly
blew out Courier New's proportions, capping it left the difference visible -
so the size is the user's call now. Ranges are clamped in the schema (the
interface size drives the root font size, and with it every rem-based
dimension) and each preview renders at the size its surface will use.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>