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>

tsconfig.base.json

@@ -0,0 +1,26 @@
+{
+  "compileOnSave": false,
+  "compilerOptions": {
+    "rootDir": ".",
+    "sourceMap": true,
+    "declaration": false,
+    "moduleResolution": "node",
+    "emitDecoratorMetadata": true,
+    "experimentalDecorators": true,
+    "importHelpers": true,
+    "target": "es2015",
+    "module": "esnext",
+    "lib": [
+      "es2020",
+      "dom"
+    ],
+    "skipLibCheck": true,
+    "skipDefaultLibCheck": true,
+    "baseUrl": ".",
+    "paths": {}
+  },
+  "exclude": [
+    "node_modules",
+    "tmp"
+  ]
+}