integration: collapse the mobile nav after hydration and keep fragment targets clear of the sticky header

SSR still ships the expanded menu, but phones collapse it after render, stop sticking the header below md, and use the header-offset token so case anchors no longer sit underneath the bar.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/app/app.scss

@@ -14,7 +14,7 @@
 }
 
 .site-header {
-  position: sticky;
+  position: relative;
   top: 0;
   z-index: 10;
   padding-block: var(--space-3);
@@ -157,6 +157,10 @@
 }
 
 @include bp.respond-to(md) {
+  .site-header {
+    position: sticky;
+  }
+
   .nav-toggle {
     display: none;
   }