Mobile menu: make the open menu scrollable (#123)

The open mobile menu is a fixed panel with every group expanded, about 930px
tall — taller than an iPhone screen — so the bottom items (Resources, Suggest a
resource) could not be reached, as a fixed element is not scrolled by the page.
Cap its height to the viewport (dvh, with a vh fallback) and let it scroll.

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

Author: Mazizova

assets/scss/_header.scss

@@ -279,6 +279,15 @@
     margin-left: 0;
     z-index: 99;
 
+    // The open menu is taller than a phone screen (every group is expanded),
+    // and a fixed element can't be scrolled by the page — so it scrolls itself.
+    max-height: calc(100vh - 64px);
+    max-height: calc(100dvh - 64px);
+    overflow-y: auto;
+    overscroll-behavior: contain;
+    -webkit-overflow-scrolling: touch;
+    padding-bottom: calc(1rem + env(safe-area-inset-bottom, 0px));
+
     &.open {
       display: flex;
     }