test(e2e): cover admin thumbnail generation card + a11y

- admin.spec: click 'Thumbnails erzeugen', wait for status DONE
  within 30s, screenshot the success message
- accessibility.spec: /admin/system joins the page list so the
  thumbnail card is checked in light, system-dark, and manual-dark
  axe-core runs

Refs #307

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/e2e/accessibility.spec.ts

@@ -11,7 +11,8 @@ const AUTHENTICATED_PAGES = [
 	{ name: 'home', path: '/' },
 	{ name: 'persons', path: '/persons' },
 	{ name: 'aktivitaeten', path: '/aktivitaeten' },
-	{ name: 'admin', path: '/admin' }
+	{ name: 'admin', path: '/admin' },
+	{ name: 'admin-system', path: '/admin/system' }
 ];
 
 function buildAxe(page: Parameters<typeof AxeBuilder>[0]['page']) {