🐛 Pin fixed headers to the viewport top explicitly

The fixed wrappers relied on their static position resolving to y=0 as
direct flex children of body; the header landmark added in #3048 made
that position fall below the spacer. Set top-0 on all fixed variants.

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

layouts/partials/header/fixed.html

@@ -1,5 +1,5 @@
 <div class="min-h-[148px]"></div>
-<div class="fixed inset-x-0 z-100">
+<div class="fixed top-0 inset-x-0 z-100">
   <div
     id="menu-blur"
     class="absolute opacity-0 inset-x-0 top-0 h-full single_hero_background nozoom backdrop-blur-2xl shadow-2xl bg-neutral/25 dark:bg-neutral-800/25"></div>