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.

lib/mv_web/live/components/member_filter_component.ex

@@ -156,6 +156,7 @@ defmodule MvWeb.Components.MemberFilterComponent do
         >
           {@member_count}
         </.badge>
+        <.icon name="hero-chevron-down" class="size-4" />
       </.button>
       
     <!--