feat(portal-self-service): self-service portal placeholder page (refs #65)

Replace the generated Nx welcome page with a minimal self-service placeholder
(Dutch 'Zelfservice — BIG-registratie' heading + router-outlet); drop nx-welcome.
The login + submit form arrive in S-08c.

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

apps/self-service/src/app/app.ts

@@ -1,9 +1,8 @@
 import { Component } from '@angular/core';
 import { RouterModule } from '@angular/router';
-import { NxWelcome } from './nx-welcome';
 
 @Component({
-  imports: [NxWelcome, RouterModule],
+  imports: [RouterModule],
   selector: 'app-root',
   templateUrl: './app.html',
   styleUrl: './app.css',