test(e2e): isolate the self-service specs with dedicated DigiD users (refs #111)

Resume-on-load (S-26) restores any open registration for the logged-in bsn, so on the
shared verify stack the specs can no longer share jan-burger: verify-domain submits as
jan-burger (123456782) before the e2e, and that open registration was being resumed on
login. Each self-service spec now uses its own citizen — registration→emma-burger,
resume→sanne-burger, withdrawal→lars-burger — none touched by the verify-* checks or
each other. jan-burger stays the documented citizen for the API-level verify checks.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/e2e/resume.spec.ts

@@ -6,7 +6,10 @@ import { expect, test } from '@playwright/test';
 test('DigiD submit → reload → self-service restores the existing registration', async ({ page }) => {
   await page.goto('/');
 
-  await page.locator('#username').fill('jan-burger');
+  // Its own DigiD user (like every self-service spec): on the shared verify stack, resume-on-load
+  // (S-26) restores any open registration for the bsn, so each spec uses a dedicated citizen that no
+  // other spec or verify-* check touches. This one in particular leaves an open registration.
+  await page.locator('#username').fill('sanne-burger');
   await page.locator('#password').fill('test123');
   await page.locator('#kc-login').click();