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>

libs/ui/project.json

@@ -0,0 +1,13 @@
+{
+  "name": "ui",
+  "$schema": "../../node_modules/nx/schemas/project-schema.json",
+  "sourceRoot": "libs/ui/src",
+  "prefix": "lib",
+  "projectType": "library",
+  "tags": [],
+  "targets": {
+    "lint": {
+      "executor": "@nx/eslint:lint"
+    }
+  }
+}