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/ToggleItemButton.tsx

@@ -3,6 +3,7 @@
 import { useState } from 'react'
 import { toggleItem } from '@/lib/actions'
 import { CheckCircle2, Circle } from 'lucide-react'
+import { useToast } from './ToastContainer'
 
 interface ToggleItemButtonProps {
   itemId: string
@@ -11,6 +12,7 @@ interface ToggleItemButtonProps {
 }
 
 export default function ToggleItemButton({ itemId, completed, variant = 'full' }: ToggleItemButtonProps) {
+  const { showToast } = useToast()
   const [isToggling, setIsToggling] = useState(false)
 
   const handleToggle = async (e: React.MouseEvent) => {
@@ -19,8 +21,10 @@ export default function ToggleItemButton({ itemId, completed, variant = 'full' }
     setIsToggling(true)
     try {
       await toggleItem(itemId)
+      showToast(completed ? 'Task marked incomplete' : 'Task completed', 'success')
     } catch (error) {
       console.error('Error toggling item:', error)
+      showToast('Failed to update task', 'error')
     } finally {
       setIsToggling(false)
     }