fix(ds): shrink dialog close button to size sm (#2309)

The dialog close button rendered as a :md icon button (44x44px with a
20px glyph) — noticeably larger than the dialog's own action buttons
(36px tall) and visually heavy next to the title. Pass size: :sm so the
close control is 32x32px with a 16px glyph, matching the action row's
weight. 32px still clears the WCAG 2.5.8 (AA) 24px minimum target.

app/components/DS/dialog.rb

@@ -142,6 +142,7 @@ class DS::Dialog < DesignSystemComponent
     classes = responsive? ? "ml-auto hidden lg:flex" : "ml-auto"
     render DS::Button.new(
       variant: "icon",
+      size: :sm,
       class: classes,
       icon: "x",
       title: I18n.t("ds.dialog.close"),