Fix mobile menu not covering viewport when nav is scrolled

backdrop-filter on #site-nav creates a CSS containing block, causing
fixed inset-0 children to be clipped to the nav bar instead of the
viewport. Portal the menu to document.body on init to escape it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/SiteHeader.astro

@@ -152,6 +152,10 @@ const lineClass = isDark ? "bg-white" : "bg-black";
     const closeButton = document.getElementById("menu-close");
 
     if (menu && openButton && closeButton) {
+        // Move menu to document.body so backdrop-filter on the nav wrapper
+        // doesn't create a new containing block that clips fixed positioning.
+        document.body.appendChild(menu);
+
         const toggleMenu = (open) => {
             menu.classList.toggle("hidden", !open);
             menu.setAttribute("aria-hidden", open ? "false" : "true");