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/catalogus.spec.ts

@@ -1,5 +1,5 @@
 import { expect, test } from '@playwright/test';
-import { loginMedewerker } from './medewerker-login';
+import { loginMedewerker } from './keycloak-login';
 
 // S-15a walking skeleton: a beheerder logs in to the beheer portal (medewerker realm) and sees the
 // read-only ZTC catalogus. The verify stack seeds and publishes the BIG-REGISTRATIE zaaktype (the