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/appearance_form.rs

@@ -8,6 +8,8 @@ use gpui::{
 };
 use gpui_component::{IconName, scroll::ScrollableElement};
 
+use super::appearance_layout_cards::render_sidebar_layout_section;
+
 use crate::shell::ElyShell;
 use crate::shell::chrome::SERIF_FAMILY;
 
@@ -31,7 +33,8 @@ pub(crate) fn render_appearance_form(
                 .gap(px(28.0))
                 .child(render_header())
                 .child(render_wallpaper_grid(snapshot, cx))
-                .child(render_appearance_rows(snapshot, cx)),
+                .child(render_appearance_rows(snapshot, cx))
+                .child(render_sidebar_layout_section(snapshot, cx)),
         )
         .into_any_element()
 }