refactor(e2e): route every portal login through one Keycloak helper (refs #161)

`medewerker-login.ts` becomes `keycloak-login.ts`: the three citizen specs
each duplicated the same three-line password login, so a fix to the login path
had to be made four times. They now call `loginBurger`, and both realms share
`submitPassword`.

No behaviour change — all 6 specs green against a live stack.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/e2e/withdrawal.spec.ts

@@ -1,4 +1,5 @@
 import { expect, test } from '@playwright/test';
+import { loginBurger } from './keycloak-login';
 
 // S-11 (Flow 3): a zorgprofessional logs in via mock DigiD, submits a registration, then withdraws
 // it ("trek aanvraag in") from the self-service portal. The withdrawal goes portal → BFF (owner-
@@ -10,9 +11,7 @@ test('DigiD submit → trek aanvraag in → self-service confirms ingetrokken',
 
   // Its own DigiD user — isolated from the verify-* checks (jan-burger/123456782) so resume-on-load
   // (S-26) can't restore someone else's registration on the shared stack.
-  await page.locator('#username').fill('lars-burger');
-  await page.locator('#password').fill('test123');
-  await page.locator('#kc-login').click();
+  await loginBurger(page, 'lars-burger');
 
   await expect(page.getByRole('heading', { name: /Zelfservice/i })).toBeVisible();