feat(web): add chevron affordance and scope-badge slot to dropdown triggers

Dropdown openers were visually indistinguishable from ordinary buttons. A
trailing chevron now marks every dropdown trigger — both the shared
dropdown_menu component and the bespoke member-filter trigger — and an
optional badge slot lets a trigger show a status indicator beside its label.

test/mv_web/components/field_visibility_dropdown_component_test.exs

@@ -17,5 +17,13 @@ defmodule MvWeb.Components.FieldVisibilityDropdownComponentTest do
       assert has_element?(view, "button[phx-click='select_all']")
       assert has_element?(view, "button[phx-click='select_none']")
     end
+
+    test "trigger carries a trailing chevron affordance", %{conn: conn} do
+      conn = conn_with_oidc_user(conn)
+      {:ok, _view, html} = live(conn, ~p"/members")
+
+      # The shared dropdown trigger signals "opens a menu" with a trailing chevron.
+      assert html =~ "hero-chevron-down"
+    end
   end
 end