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.

app/components/DS/menu.html.erb

@@ -1,6 +1,6 @@
 <%= tag.div data: { controller: "DS--menu", DS__menu_placement_value: placement, DS__menu_offset_value: offset, DS__menu_mobile_fullwidth_value: mobile_fullwidth, testid: testid } do %>
-  <% if variant == :icon %>
-    <%= render DS::Button.new(variant: "icon", icon: icon_vertical ? "more-vertical" : "more-horizontal", aria: { haspopup: "menu", expanded: "false", controls: menu_id }, data: { DS__menu_target: "button" }) %>
+  <% if icon_only? %>
+    <%= render DS::Button.new(variant: "icon", size: icon_button_size, icon: icon_vertical ? "more-vertical" : "more-horizontal", aria: { haspopup: "menu", expanded: "false", controls: menu_id }, data: { DS__menu_target: "button" }) %>
   <% elsif variant == :button %>
     <%= button %>
   <% end %>