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

components/SkeletonLoader.tsx

@@ -0,0 +1,76 @@
+'use client'
+
+interface SkeletonProps {
+  className?: string
+}
+
+export function Skeleton({ className = '' }: SkeletonProps) {
+  return (
+    <div className={`animate-pulse bg-gray-200 rounded ${className}`} />
+  )
+}
+
+export function ListCardSkeleton() {
+  return (
+    <div className="bg-white rounded-lg shadow-sm border border-gray-200 p-5 sm:p-6 min-h-[140px] sm:min-h-[160px]">
+      <div className="flex items-start justify-between mb-4 gap-2">
+        <div className="flex items-center gap-2 sm:gap-3 flex-1">
+          <Skeleton className="w-3 h-3 sm:w-4 sm:h-4 rounded-full" />
+          <Skeleton className="h-6 w-32 sm:w-40" />
+        </div>
+      </div>
+      
+      <div className="space-y-2.5 sm:space-y-3">
+        <div className="flex items-center justify-between">
+          <Skeleton className="h-4 w-24" />
+          <Skeleton className="h-4 w-8" />
+        </div>
+        <Skeleton className="h-2 sm:h-2.5 w-full rounded-full" />
+      </div>
+    </div>
+  )
+}
+
+export function ItemCardSkeleton() {
+  return (
+    <div className="bg-white rounded-lg border border-gray-200 p-4 sm:p-5">
+      <div className="flex items-start gap-3 sm:gap-4">
+        <Skeleton className="w-5 h-5 sm:w-6 sm:h-6 rounded-full flex-shrink-0 mt-0.5 sm:mt-1" />
+        
+        <div className="flex-1 min-w-0 space-y-2">
+          <Skeleton className="h-5 w-3/4" />
+          <Skeleton className="h-4 w-full" />
+          <Skeleton className="h-4 w-2/3" />
+          
+          <div className="flex flex-wrap items-center gap-1.5 sm:gap-2 mt-2">
+            <Skeleton className="h-6 w-16 rounded" />
+            <Skeleton className="h-6 w-20 rounded" />
+            <Skeleton className="h-6 w-24 rounded" />
+          </div>
+        </div>
+        
+        <Skeleton className="w-5 h-5 rounded flex-shrink-0" />
+      </div>
+    </div>
+  )
+}
+
+export function ListDetailSkeleton() {
+  return (
+    <div className="space-y-4">
+      <ItemCardSkeleton />
+      <ItemCardSkeleton />
+      <ItemCardSkeleton />
+    </div>
+  )
+}
+
+export function SearchResultsSkeleton() {
+  return (
+    <div className="space-y-3">
+      <Skeleton className="h-5 w-32" />
+      <ItemCardSkeleton />
+      <ItemCardSkeleton />
+    </div>
+  )
+}