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/ScenariosPage.tsx

@@ -5,12 +5,13 @@ import { Play, Plus, Trash2, Download, ClipboardList } from 'lucide-react';
 import { parse as yamlParse } from 'yaml';
 import { scenarios } from '../../api';
 import type { Scenario } from '../../api';
-import { Breadcrumbs, Button, Table, type TableColumn, Timestamp, UuidBadge } from '../../ui';
+import { Breadcrumbs, Button, Table, type TableColumn, Timestamp, UuidBadge, useToast } from '../../ui';
 import styles from '../Page.module.css';
 
 export function ScenariosPage() {
   const { t } = useTranslation();
   const navigate = useNavigate();
+  const toast = useToast();
   const [items, setItems] = useState<Scenario[]>([]);
   const [loading, setLoading] = useState(true);
   const [error, setError] = useState<string | null>(null);
@@ -30,8 +31,13 @@ export function ScenariosPage() {
   }, [load]);
 
   const handleRun = async (id: string) => {
-    const run = await scenarios.run(id);
-    navigate(`/scenarios/${id}/runs/${run.id}`);
+    try {
+      const run = await scenarios.run(id);
+      toast.success('Scenario run started');
+      navigate(`/scenarios/${id}/runs/${run.id}`);
+    } catch (err) {
+      toast.error((err as Error).message);
+    }
   };
 
   const handleDelete = async (id: string) => {