Below the appearance rows the Appearance form now exposes the design's
"Sidebar / Layout" section: serif "Layout" sub-headline plus a 2-up
preview-card grid wired to the active space's sidebar_width — Single
column lands on DEFAULT_SIDEBAR_WIDTH_PX, Compact lands on
COLLAPSED_SIDEBAR_WIDTH_PX. Each card draws a miniature sidebar +
canvas preview so the choice reads at a glance.
The shell exposes a new set_active_sidebar_width helper alongside the
existing toggle so the cards mutate state without re-implementing the
toggle logic. The "Hidden on hover" preset from the design is omitted
until that mode actually ships — no placeholder option.
The cards live in chrome::appearance_layout_cards so appearance_form
stays under 500 lines.
ZacharyZhang-NY
20f3740089