fix: fix admin mobile nav overflow breaking layout on small screens

Mobile nav now scrolls horizontally with hidden scrollbar; nav items
don't shrink and show icon-only on xs, icon+label on sm and up.
Added scrollbar-none utility to app.css.

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

packages/frontend/src/app.css

@@ -3,6 +3,13 @@
 
 @plugin "@iconify/tailwind4";
 
+@utility scrollbar-none {
+  scrollbar-width: none;
+  &::-webkit-scrollbar {
+    display: none;
+  }
+}
+
 @custom-variant dark (&:where(.dark, .dark *));
 
 @custom-variant hover (&:hover);