feat(categories): add schema image thumbnails to grid and tree views

Enrich category children with batch-fetched schema images and leaf detection
on the API side. Display thumbnails in grid cards with shimmer loading
placeholders and background prefetching. Add TanStack Query caching for
category children. Fix insert race condition with onConflictDoNothing.

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

apps/web/src/globals.css

@@ -37,6 +37,11 @@
   }
 }
 
+@keyframes shimmer {
+  0% { transform: translateX(-100%); }
+  100% { transform: translateX(100%); }
+}
+
 .dark {
   --color-background: #0a0a0a;
   --color-foreground: #fafafa;