integration: return HTTP 404 for unmatched URLs and close remaining browser-gate gaps

Co-authored-by: Cursor <cursoragent@cursor.com>

e2e/seo.e2e.ts

@@ -30,6 +30,14 @@ test.describe('server-rendered metadata', () => {
     }
   });
 
+  test('unmatched URLs return 404 and a real route stays 200', async ({ request }) => {
+    const home = await request.get('/');
+    expect(home.status(), 'GET /').toBe(200);
+
+    await expectHead(request, 'notFound', 'de');
+    await expectHead(request, 'notFound', 'en');
+  });
+
   test('client navigation does not duplicate head tags', async ({ page }) => {
     await page.goto('/');
     await page.locator('a.contact-cta').first().waitFor();