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/resume.spec.ts

@@ -1,4 +1,5 @@
 import { expect, test } from '@playwright/test';
+import { loginBurger } from './keycloak-login';
 
 // S-26: a zorgprofessional submits, then reloads the self-service portal. On load the portal asks the
 // BFF for the caller's current open registration (owner-scoped by the DigiD token's bsn) and restores
@@ -9,9 +10,7 @@ test('DigiD submit → reload → self-service restores the existing registratio
   // 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();
+  await loginBurger(page, 'sanne-burger');
 
   await expect(page.getByRole('heading', { name: /Zelfservice/i })).toBeVisible();
   await page.getByRole('button', { name: /indienen/i }).click();