fix(design-system): DS::Menu add :icon_sm variant for dense action lists (#1930)

PR #1840 bumped DS::Button icon-only `:md` size from `w-9 h-9` (36×36)
to `w-11 h-11` (44×44) for WCAG 2.5.5 enhanced touch target. DS::Menu's
`:icon` variant uses DS::Button at the default `:md` size, so every
row-level "..." action-list trigger grew from 36×36 to 44×44.

For dense lists where each row has a trigger — most visibly the
transaction category dropdown (`category/dropdowns/_row.html.erb`) —
the per-row height bump (+8px) compounds: a 5-category panel that
used to fit in ~220px now wants ~260px, the badges look smaller
relative to the row chrome, and the overall density that made the
dropdown scannable regresses visibly.

Add an `:icon_sm` variant that renders the trigger as DS::Button at
`size: :sm` (32×32). Meets WCAG 2.5.8 AA (24×24) — appropriate for
compact in-row triggers where 44×44 isn't required. Standalone
toolbar / row-action `...` triggers should keep `:icon` for AAA.

Migrate `category/dropdowns/_row.html.erb` to `:icon_sm` to restore
the pre-#1840 dropdown density.

test/components/previews/menu_component_preview.rb

@@ -5,6 +5,12 @@ class MenuComponentPreview < ViewComponent::Preview
     end
   end
 
+  def icon_sm
+    render DS::Menu.new(variant: "icon_sm") do |menu|
+      menu_contents(menu)
+    end
+  end
+
   def button
     render DS::Menu.new(variant: "button") do |menu|
       menu.with_button(text: "Open menu", variant: "secondary")