feat(ui): add toasts for create edit and run flows

- add global ToastProvider and useToast hook mounted at app root

- show success/error toasts for environment, credential, snippet, and scenario create/edit actions

- show run-started/error toasts on scenario run actions with 100ms exit animation

client/src/pages/scenario/ScenarioDetailPage.tsx

@@ -16,6 +16,7 @@ import {
   Table,
   Timestamp,
   UuidBadge,
+  useToast,
   type TableColumn,
 } from '../../ui';
 import styles from '../Page.module.css';
@@ -24,6 +25,7 @@ export function ScenarioDetailPage() {
   const { t } = useTranslation();
   const { id } = useParams<{ id: string }>();
   const navigate = useNavigate();
+  const toast = useToast();
   const [scenario, setScenario] = useState<(Scenario & { steps: ScenarioStep[] }) | null>(null);
   const [loading, setLoading] = useState(true);
   const [error, setError] = useState<string | null>(null);
@@ -66,8 +68,13 @@ export function ScenarioDetailPage() {
 
   const handleRun = async () => {
     if (!scenario) return;
-    const run = await scenarios.run(scenario.id);
-    navigate(`/scenarios/${id}/runs/${run.id}`);
+    try {
+      const run = await scenarios.run(scenario.id);
+      toast.success('Scenario run started');
+      navigate(`/scenarios/${id}/runs/${run.id}`);
+    } catch (err) {
+      toast.error((err as Error).message);
+    }
   };
 
   const handleDelete = async () => {