test(briefwechsel): guard the removed /briefwechsel route returns 404

Add an active e2e spec asserting /briefwechsel 404s on the styled app
error page. The old assertion lived in stammbaum.spec.ts inside a
test.skip() block (never executed) and asserted the opposite — remove it.
Drop /briefwechsel from the auth protected-route loop; /documents (the
redirect target) sits behind the same authenticated() rule, so coverage
is preserved.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/e2e/auth.spec.ts

@@ -26,7 +26,7 @@ test.describe('Authentication', () => {
 	});
 
 	test('protected routes redirect to /login without session', async ({ page }) => {
-		for (const url of ['/documents/new', '/persons', '/briefwechsel']) {
+		for (const url of ['/documents/new', '/persons']) {
 			await page.goto(url);
 			await expect(page).toHaveURL(/\/login/);
 		}