feat: integrate NotificationStack into App

client/src/app/App.tsx

@@ -2,6 +2,7 @@ import { BrowserRouter } from 'react-router-dom'
 import { AppProviders } from '@/app/providers/AppProviders'
 import { AppRoutes } from '@/app/routes'
 import { CartSnackbar } from '@/shared/ui/CartSnackbar'
+import { NotificationStack } from '@/shared/ui/NotificationStack'
 import { ErrorBoundary } from '@/shared/ui/ErrorBoundary'
 import { NoiseOverlay } from '@/shared/ui/NoiseOverlay'
 
@@ -13,6 +14,7 @@ export function App() {
           <AppRoutes />
         </ErrorBoundary>
         <CartSnackbar />
+        <NotificationStack />
         <NoiseOverlay />
       </BrowserRouter>
     </AppProviders>