The custom input now replaces the dropdown instead of stacking beneath it,
so entering custom mode no longer grows the row and pushes the rest of the
page down; it autofocuses, and Escape discards uncommitted typing without
closing the settings page.

Typefaces differ in x-height at the same pixel size (DM Sans 0.51 vs
JetBrains Mono 0.55), so switching family also changed apparent text size.
Each surface now carries a font-size-adjust measured from its own default
stack: the platform default renders unchanged, and any chosen font is
scaled to match it. Measurements are re-taken once webfonts finish loading,
since an early probe describes the fallback face rather than the default.

The sidebar PR number follows the interface font (tabular digits keep it
from reflowing) instead of the code font.

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