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/main.tsx

@@ -4,11 +4,14 @@ import { HashRouter } from 'react-router-dom';
 import './i18n';
 import './ui/tokens.css';
 import App from './App';
+import { ToastProvider } from './ui';
 
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
-    <HashRouter>
-      <App />
-    </HashRouter>
+    <ToastProvider>
+      <HashRouter>
+        <App />
+      </HashRouter>
+    </ToastProvider>
   </StrictMode>,
 );