Add Sidebar / Layout cards to appearance form

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.

crates/ely_app/src/shell/chrome/mod.rs

@@ -1,4 +1,5 @@
 pub(crate) mod appearance_form;
+pub(crate) mod appearance_layout_cards;
 pub(crate) mod brand_glyph;
 pub(crate) mod command_footer;
 pub(crate) mod command_overlay;