refactor: replace Toast with Sonner

frontend/app/app.css

@@ -196,10 +196,6 @@
     animation: modal-in 0.18s ease-out;
   }
 
-  .toast-enter {
-    animation: toast-in 0.2s ease-out;
-  }
-
   .notebook-md h1,
   .notebook-md h2,
   .notebook-md h3,
@@ -260,13 +256,6 @@
   }
 }
 
-@keyframes toast-in {
-  from {
-    opacity: 0;
-    transform: translateY(-7px);
-  }
-}
-
 @keyframes spin {
   to {
     transform: rotate(360deg);