🐛 Let the fullscreen mobile menu escape the floating header

The floating header's backdrop blur makes the pill a containing block
for fixed-position descendants, so the fullscreen mobile menu rendered
at the pill's height instead of filling the viewport. Lift the filter
while the menu is open. Fixes #3057.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/css/compiled/main.css

@@ -4900,6 +4900,10 @@ body:has(#mobile-menu-toggle:checked) {
   visibility: hidden;
   transition: visibility 0.3s, opacity 0.3s ease-in-out;
 }
+header > div:has(.mobile-menu-fullscreen #mobile-menu-toggle:checked) {
+  -webkit-backdrop-filter: none;
+  backdrop-filter: none;
+}
 .nested-menu:nth-last-child(-n + 3) .menuhide, .mobile-header-row .menuhide {
   inset-inline-end: 0;
 }