test(portal-self-service): self-service portal placeholder renders (refs #65)

Bootstrap the Nx (pnpm) workspace at the repo root with the self-service Angular
app (standalone + signals, Vitest via @angular/build, ESLint) — the frontend
foundation. Nx is scoped to apps/+libs/ only; the .NET services stay on
dotnet/Makefile (no @nx/docker inference). A failing test asserts the app renders
a 'Zelfservice' heading; it still shows the generated Nx welcome page, so it fails.
Green commit implements the placeholder.

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

eslint.config.mjs

@@ -0,0 +1,58 @@
+import nx from '@nx/eslint-plugin';
+
+export default [
+  ...nx.configs['flat/base'],
+  ...nx.configs['flat/typescript'],
+  ...nx.configs['flat/javascript'],
+  {
+    ignores: [
+      '**/dist',
+      '**/vite.config.*.timestamp*',
+      '**/vitest.config.*.timestamp*',
+    ],
+  },
+  {
+    files: ['**/*.ts', '**/*.tsx', '**/*.js', '**/*.jsx'],
+    rules: {
+      '@nx/enforce-module-boundaries': [
+        'error',
+        {
+          enforceBuildableLibDependency: true,
+          allow: ['^.*/eslint(\\.base)?\\.config\\.[cm]?[jt]s$'],
+          depConstraints: [
+            {
+              sourceTag: 'scope:shared',
+              onlyDependOnLibsWithTags: ['scope:shared'],
+            },
+            {
+              sourceTag: 'scope:shop',
+              onlyDependOnLibsWithTags: ['scope:shop', 'scope:shared'],
+            },
+            {
+              sourceTag: 'scope:api',
+              onlyDependOnLibsWithTags: ['scope:api', 'scope:shared'],
+            },
+            {
+              sourceTag: 'type:data',
+              onlyDependOnLibsWithTags: ['type:data'],
+            },
+          ],
+        },
+      ],
+    },
+  },
+  {
+    files: [
+      '**/*.ts',
+      '**/*.tsx',
+      '**/*.cts',
+      '**/*.mts',
+      '**/*.js',
+      '**/*.jsx',
+      '**/*.cjs',
+      '**/*.mjs',
+    ],
+    // Override or add rules here
+    rules: {},
+  },
+];