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/ListDetailView.tsx

@@ -5,6 +5,7 @@ import ItemCard from './ItemCard'
 import SearchBar from './SearchBar'
 import FilterPanel from './FilterPanel'
 import SearchResults from './SearchResults'
+import EmptyState from './EmptyState'
 import { searchItems, getItemsByFilter } from '@/lib/actions'
 import { useSearch, useFilters, useScrollPosition } from '@/hooks'
 
@@ -138,7 +139,7 @@ export default function ListDetailView({ listId, initialItems }: ListDetailViewP
         <div className="space-y-6">
           {/* Incomplete Items */}
           {incompleteItems.length > 0 && (
-            <div>
+            <div className="animate-fade-in">
               <h2 className="text-lg font-semibold text-gray-900 mb-3">
                 To Do ({incompleteItems.length})
               </h2>
@@ -152,7 +153,7 @@ export default function ListDetailView({ listId, initialItems }: ListDetailViewP
 
           {/* Completed Items */}
           {completedItems.length > 0 && (
-            <div>
+            <div className="animate-fade-in">
               <h2 className="text-lg font-semibold text-gray-500 mb-3">
                 Completed ({completedItems.length})
               </h2>
@@ -166,14 +167,15 @@ export default function ListDetailView({ listId, initialItems }: ListDetailViewP
 
           {/* Empty State */}
           {displayItems.length === 0 && (
-            <div className="text-center py-12 bg-gray-50 rounded-lg">
-              <p className="text-gray-500 text-lg mb-2">No tasks found</p>
-              <p className="text-gray-400">
-                {filters.priority || filters.completed !== null || filters.tags.length > 0
-                  ? 'Try adjusting your filters'
-                  : 'Add your first task to get started'}
-              </p>
-            </div>
+            <EmptyState
+              icon={filters.priority || filters.completed !== null || filters.tags.length > 0 ? 'filter' : 'inbox'}
+              title="No tasks found"
+              description={
+                filters.priority || filters.completed !== null || filters.tags.length > 0
+                  ? 'Try adjusting your filters to see more tasks'
+                  : 'Add your first task using the button above'
+              }
+            />
           )}
         </div>
       )}