test(03-04): add failing tests for RemoteConfigStep

- BACK-01: Azure Blob form with account field + AzureAuthToggle
- BACK-02: S3 form fields (access_key_id, secret_access_key, region)
- BACK-03: S3-compatible form includes endpoint field
- renderWithBackend helper dispatches SET_BACKEND_TYPE before render
- @testing-library/user-event installed for value retention test

package.json

@@ -21,6 +21,7 @@
     "@tailwindcss/vite": "^4.2.2",
     "@testing-library/dom": "^10.4.1",
     "@testing-library/react": "^16.3.2",
+    "@testing-library/user-event": "^14.6.1",
     "@types/react": "^18.3.5",
     "@types/react-dom": "^18.3.0",
     "@vitejs/plugin-react": "^4.7.0",