The sticky rework had swapped the toolbar background to opaque
bg-muted, which reads darker than main's bg-muted/30. Restore the
translucent background and pair it with backdrop-blur + a slightly
stronger muted overlay (only when backdrop-filter is supported) so
pinned-on-body-text stays legible without the flat solid band.
Also pin the font-size button's line height so the toolbar's row
height matches main exactly.
Co-Authored-By: Claude <noreply@anthropic.com>