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/eslint.config.mjs

@@ -0,0 +1,34 @@
+import nx from '@nx/eslint-plugin';
+import baseConfig from '../../eslint.config.mjs';
+
+export default [
+  ...nx.configs['flat/angular'],
+  ...nx.configs['flat/angular-template'],
+  ...baseConfig,
+  {
+    files: ['**/*.ts'],
+    rules: {
+      '@angular-eslint/directive-selector': [
+        'error',
+        {
+          type: 'attribute',
+          prefix: 'lib',
+          style: 'camelCase',
+        },
+      ],
+      '@angular-eslint/component-selector': [
+        'error',
+        {
+          type: 'element',
+          prefix: 'lib',
+          style: 'kebab-case',
+        },
+      ],
+    },
+  },
+  {
+    files: ['**/*.html'],
+    // Override or add rules here
+    rules: {},
+  },
+];