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/snippet/CreateSnippetPage.tsx

@@ -3,12 +3,13 @@ import { useNavigate } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
 import { X, Save, Braces } from 'lucide-react';
 import { snippets } 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 CreateSnippetPage() {
   const { t } = useTranslation();
   const navigate = useNavigate();
+  const toast = useToast();
 
   const [name, setName] = useState('');
   const [description, setDescription] = useState('');
@@ -38,9 +39,12 @@ export function CreateSnippetPage() {
         description: description.trim() || undefined,
         code: code.trim(),
       });
+      toast.success('Snippet created');
       navigate(`/snippets/${snippet.id}`);
     } catch (err) {
-      setError((err as Error).message);
+      const message = (err as Error).message;
+      setError(message);
+      toast.error(message);
     } finally {
       setSaving(false);
     }

client/src/pages/snippet/EditSnippetPage.tsx

@@ -4,13 +4,14 @@ import { useTranslation } from 'react-i18next';
 import { X, Save, Braces } from 'lucide-react';
 import { snippets } from '../../api';
 import type { Snippet } 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 EditSnippetPage() {
   const { t } = useTranslation();
   const { id } = useParams<{ id: string }>();
   const navigate = useNavigate();
+  const toast = useToast();
 
   const [snippet, setSnippet] = useState<Snippet | null>(null);
   const [name, setName] = useState('');
@@ -56,9 +57,12 @@ export function EditSnippetPage() {
         description: description.trim() || undefined,
         code: code.trim(),
       });
+      toast.success('Snippet updated');
       navigate(`/snippets/${id}`);
     } catch (err) {
-      setError((err as Error).message);
+      const message = (err as Error).message;
+      setError(message);
+      toast.error(message);
     } finally {
       setSaving(false);
     }