test(search): assert lang field sent in E2E NL search request

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/e2e/nl-search.spec.ts

@@ -37,6 +37,8 @@ test.describe('NL (smart) search — happy path', () => {
 	}) => {
 		// Deliberate delay so the loading state is assertable before the response arrives.
 		await page.route('**/api/search/nl', async (route) => {
+			const body = route.request().postDataJSON();
+			expect(body.lang).toBeTruthy();
 			await new Promise((resolve) => setTimeout(resolve, 150));
 			await route.fulfill({
 				status: 200,