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/CreateScenarioPage.tsx

@@ -3,12 +3,13 @@ import { useNavigate } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
 import { X, Save, ClipboardList } from 'lucide-react';
 import { scenarios } from '../../api';
-import { Breadcrumbs, Button, Card, Input } from '../../ui';
+import { Breadcrumbs, Button, Card, Input, useToast } from '../../ui';
 import styles from '../Page.module.css';
 
 export function CreateScenarioPage() {
   const { t } = useTranslation();
   const navigate = useNavigate();
+  const toast = useToast();
 
   const [name, setName] = useState('');
   const [nameError, setNameError] = useState('');
@@ -25,9 +26,12 @@ export function CreateScenarioPage() {
     setError(null);
     try {
       const scenario = await scenarios.create(name.trim());
+      toast.success('Scenario created');
       navigate(`/scenarios/${scenario.id}`);
     } catch (err) {
-      setError((err as Error).message);
+      const message = (err as Error).message;
+      setError(message);
+      toast.error(message);
     } finally {
       setSaving(false);
     }