Fix mobile sidebar overlap - hide Collections pane on mobile

Issue: Collections sidebar was overlapping main content on mobile screens
Fix: Hide sidebar completely on mobile (display: none)

Mobile users navigate via the top navigation/breadcrumbs instead.
Content now takes full width without sidebar overlap.

admin/index.html

@@ -22,6 +22,13 @@
     ═══════════════════════════════════════════════════════════ */
     
     @media (max-width: 768px) {
+      /* Sidebar - HIDDEN by default on mobile */
+      aside,
+      [class*="Sidebar"],
+      nav[class*="Sidebar"] {
+        display: none !important;
+      }
+      
       /* Main content takes full width */
       main,
       [class*="MainContent"],