feat: Add visual feedback and polish (Task 15)

- Implement toast notification system for success/error messages
- Add loading states with skeleton loaders for async operations
- Create smooth CSS transitions and animations
- Enhance empty states with EmptyState component
- Add loading.tsx files for Next.js loading states
- Integrate toast notifications across all CRUD operations
- Add fade-in and scale-in animations for better UX
- Improve visual feedback for user actions

app/loading.tsx

@@ -0,0 +1,24 @@
+import { ListCardSkeleton } from '@/components/SkeletonLoader'
+
+export default function Loading() {
+  return (
+    <main className="container mx-auto px-4 sm:px-6 py-6 sm:py-8 max-w-7xl">
+      <div className="flex flex-col sm:flex-row items-start sm:items-center justify-between gap-4 mb-6 sm:mb-8">
+        <div className="h-10 w-48 bg-gray-200 rounded animate-pulse" />
+        <div className="h-12 w-32 bg-gray-200 rounded-lg animate-pulse" />
+      </div>
+
+      {/* Global Search Skeleton */}
+      <div className="mb-6 sm:mb-8">
+        <div className="h-12 bg-gray-100 border border-gray-300 rounded-lg animate-pulse" />
+      </div>
+
+      <div className="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 xl:grid-cols-4 gap-4 sm:gap-6">
+        <ListCardSkeleton />
+        <ListCardSkeleton />
+        <ListCardSkeleton />
+        <ListCardSkeleton />
+      </div>
+    </main>
+  )
+}