integration: collapse the mobile nav after hydration and keep fragment targets clear of the sticky header

SSR still ships the expanded menu, but phones collapse it after render, stop sticking the header below md, and use the header-offset token so case anchors no longer sit underneath the bar.

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

e2e/seo.e2e.ts

@@ -1,7 +1,15 @@
-import { expect, test } from '@playwright/test';
+import { expect, test, type Page } from '@playwright/test';
 import { SITE_CONTENT_DATA } from '../src/app/core/content/site-content';
 import { expectHead, jsonLdGraph, pagePath } from './helpers';
 
+async function ensurePrimaryNavOpen(page: Page): Promise<void> {
+  const toggle = page.locator('.nav-toggle');
+  if ((await toggle.isVisible()) && (await toggle.getAttribute('aria-expanded')) === 'false') {
+    await toggle.click();
+    await expect(toggle).toHaveAttribute('aria-expanded', 'true');
+  }
+}
+
 test.describe('server-rendered metadata', () => {
   test('German and English Home include the full head contract', async ({ request, page }) => {
     await expectHead(request, 'home', 'de');
@@ -40,6 +48,7 @@ test.describe('server-rendered metadata', () => {
 
   test('client navigation does not duplicate head tags', async ({ page }) => {
     await page.goto('/');
+    await ensurePrimaryNavOpen(page);
     await page.locator('a.contact-cta').first().waitFor();
 
     const assertUnique = async (canonical: string, description: string) => {
@@ -57,6 +66,7 @@ test.describe('server-rendered metadata', () => {
       );
     };
 
+    await ensurePrimaryNavOpen(page);
     await page
       .locator(`a[href="${pagePath('projects', 'de')}"]`)
       .first()
@@ -67,6 +77,7 @@ test.describe('server-rendered metadata', () => {
       SITE_CONTENT_DATA.de.pages.projects.description,
     );
 
+    await ensurePrimaryNavOpen(page);
     await page
       .locator(`a[href="${pagePath('servicesAi', 'de')}"]`)
       .first()