Header sticky: darken logo on scroll + white dropdown submenu

The light-glass sticky pill exposed two theme assumptions (it expected a dark
scrolled header):
- logo widget emits only the light logo (server-side, by light_ui); darken it
  via CSS filter on .mil-scroll/.mil-active so it's visible on the light pill
- theme sets scrolled submenu bg dark (rgba(36,47,53,.95)); make submenus a
  white panel with ink links (emerald on hover) in every state

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
