fix overlay backdrop only showing on mobile

The dark backdrop when opening the sidebar should only appear on mobile
where the sidebar overlays content, not on desktop where it slides inline.

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

index.html

@@ -148,9 +148,8 @@
       background: rgba(0, 0, 0, 0.6);
       z-index: 99;
     }
-    .sidebar-overlay.active { display: block; }
-
     @media (max-width: 640px) {
+      .sidebar-overlay.active { display: block; }
       #sidebar {
         position: fixed;
         top: 0;