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/EmptyState.tsx

@@ -0,0 +1,41 @@
+'use client'
+
+import { ReactNode } from 'react'
+import { Inbox, ListTodo, Search, Filter } from 'lucide-react'
+
+interface EmptyStateProps {
+  icon?: 'inbox' | 'list' | 'search' | 'filter'
+  title: string
+  description?: string
+  action?: ReactNode
+}
+
+export default function EmptyState({ icon = 'inbox', title, description, action }: EmptyStateProps) {
+  const icons = {
+    inbox: <Inbox size={48} className="text-gray-300" />,
+    list: <ListTodo size={48} className="text-gray-300" />,
+    search: <Search size={48} className="text-gray-300" />,
+    filter: <Filter size={48} className="text-gray-300" />,
+  }
+
+  return (
+    <div className="flex flex-col items-center justify-center py-12 sm:py-16 px-4 text-center">
+      <div className="mb-4 sm:mb-6 opacity-50">
+        {icons[icon]}
+      </div>
+      <h3 className="text-lg sm:text-xl font-semibold text-gray-700 mb-2">
+        {title}
+      </h3>
+      {description && (
+        <p className="text-sm sm:text-base text-gray-500 mb-6 max-w-md">
+          {description}
+        </p>
+      )}
+      {action && (
+        <div className="mt-4">
+          {action}
+        </div>
+      )}
+    </div>
+  )
+}