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/ItemForm.tsx

@@ -3,6 +3,7 @@
 import { useState } from 'react'
 import { useRouter } from 'next/navigation'
 import { updateItem, createItem } from '@/lib/actions'
+import { useToast } from './ToastContainer'
 
 interface ItemFormProps {
   listId: string
@@ -20,6 +21,7 @@ interface ItemFormProps {
 
 export default function ItemForm({ listId, item, mode = 'edit' }: ItemFormProps) {
   const router = useRouter()
+  const { showToast } = useToast()
   const [isSubmitting, setIsSubmitting] = useState(false)
   const [errors, setErrors] = useState<Record<string, string>>({})
 
@@ -79,14 +81,17 @@ export default function ItemForm({ listId, item, mode = 'edit' }: ItemFormProps)
           tags: data.tags,
           notes: data.notes,
         })
+        showToast('Task created successfully', 'success')
         router.push(`/list/${listId}`)
       } else if (item) {
         await updateItem(item.id, data)
+        showToast('Task updated successfully', 'success')
         router.push(`/list/${listId}/item/${item.id}`)
       }
     } catch (error) {
       console.error('Error saving item:', error)
       setErrors({ submit: 'Failed to save item. Please try again.' })
+      showToast('Failed to save task', 'error')
     } finally {
       setIsSubmitting(false)
     }