🐛 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>

layouts/partials/header/components/mobile-menu.html

@@ -1,5 +1,5 @@
 {{ $mobileMenuStyle := .Site.Params.header.mobileMenuStyle | default "fullscreen" }}
-<div class="mobile-header-row relative flex items-center h-14 gap-4">
+<div class="mobile-header-row{{ if eq $mobileMenuStyle "fullscreen" }} mobile-menu-fullscreen{{ end }} relative flex items-center h-14 gap-4">
   {{ if hugo.IsMultilingual }}
     {{ partial "header/components/translations.html" . }}
   {{ end }}