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']) {

frontend/e2e/admin.spec.ts

@@ -248,3 +248,28 @@ test.describe('Admin system tab — backfill file hashes', () => {
 		await page.screenshot({ path: 'test-results/e2e/admin-backfill-hashes.png' });
 	});
 });
+
+// ─── System tab — generate thumbnails ─────────────────────────────────────────
+
+test.describe('Admin system tab — generate thumbnails', () => {
+	test('admin triggers thumbnail generation and sees DONE within 30s', async ({ page }) => {
+		test.setTimeout(45_000);
+
+		await page.goto('/admin');
+		await page.waitForSelector('[data-hydrated]');
+
+		// Navigate to System tab
+		await page.getByRole('button', { name: /system/i }).click();
+
+		const btn = page
+			.locator('[data-thumbnails-trigger]')
+			.or(page.getByRole('button', { name: /thumbnails erzeugen/i }));
+		await expect(btn.first()).toBeVisible();
+		await btn.first().click();
+
+		// Status transitions RUNNING → DONE; poll message shows the final summary
+		await expect(page.getByTestId('thumbnails-status-done')).toBeVisible({ timeout: 30_000 });
+
+		await page.screenshot({ path: 'test-results/e2e/admin-generate-thumbnails.png' });
+	});
+});