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/registration.spec.ts

@@ -1,5 +1,5 @@
 import { expect, request, test } from '@playwright/test';
-import { loginMedewerker } from './medewerker-login';
+import { loginBurger, loginMedewerker } from './keycloak-login';
 
 // Walking-skeleton happy path (S-08d + S-09 + S-09b + S-12 + S-10a + S-19b-2): a zorgprofessional
 // logs in via mock DigiD and submits through the self-service portal → BFF → domain; the entry
@@ -23,9 +23,7 @@ test('DigiD submit → public INGEDIEND → documenten → behandelaar goedkeurt
   // checks submit as jan-burger (bsn 123456782) before the e2e runs on the shared stack, and
   // resume-on-load (S-26) would otherwise restore one of those on login — so each self-service spec
   // uses a dedicated citizen no other actor touches.
-  await page.locator('#username').fill('emma-burger');
-  await page.locator('#password').fill('test123');
-  await page.locator('#kc-login').click();
+  await loginBurger(page, 'emma-burger');
 
   // Back on the portal, authenticated.
   await expect(page.getByRole('heading', { name: /Zelfservice/i })).toBeVisible();