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/EditStepPage.tsx

@@ -4,13 +4,14 @@ import { useTranslation } from 'react-i18next';
 import { X, Save, ClipboardList } from 'lucide-react';
 import { scenarios, steps } from '../../api';
 import type { Scenario, ScenarioStep } from '../../api';
-import { Breadcrumbs, Button, Card, Input, CodeEditor } from '../../ui';
+import { Breadcrumbs, Button, Card, Input, CodeEditor, useToast } from '../../ui';
 import styles from '../Page.module.css';
 
 export function EditStepPage() {
   const { t } = useTranslation();
   const { id, stepId } = useParams<{ id: string; stepId: string }>();
   const navigate = useNavigate();
+  const toast = useToast();
 
   const [scenario, setScenario] = useState<Scenario | null>(null);
   const [step, setStep] = useState<ScenarioStep | null>(null);
@@ -45,9 +46,12 @@ export function EditStepPage() {
         execCode: execCode.trim() || undefined,
         validateCode: validateCode.trim() || undefined,
       });
+      toast.success('Step updated');
       navigate(`/scenarios/${id}`);
     } catch (err) {
-      setError((err as Error).message);
+      const message = (err as Error).message;
+      setError(message);
+      toast.error(message);
     } finally {
       setSaving(false);
     }