test(portal-self-service): DigiD-guarded registration submit page (refs #67)

Scaffold libs/ui (NL Design System via Utrecht components) and libs/auth (DigiD OIDC
over angular-auth-oidc-client: mockable AuthService, provider, token interceptor,
authenticated guard). Failing component + axe tests for the RegistrationPage: it must
show the signed-in BSN, submit to the BFF (mocked api-client) and confirm, with no
WCAG 2.1 AA violations. The page is a stub, so the behaviour tests fail; green follows.

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

tsconfig.base.json

@@ -15,7 +15,9 @@
     "skipDefaultLibCheck": true,
     "baseUrl": ".",
     "paths": {
-      "api-client": ["./libs/api-client/src/index.ts"]
+      "api-client": ["./libs/api-client/src/index.ts"],
+      "ui": ["./libs/ui/src/index.ts"],
+      "auth": ["./libs/auth/src/index.ts"]
     }
   },
   "exclude": ["node_modules", "tmp"]