Add /concepts showcase page

A teaching page pairing each pattern's impossible-state-permitting "before"
with the "after" the type system enforces: discriminated unions, the
RemoteData fold, parse-don't-validate (live), and the wizard state machine.
Composition-only — no new atoms. Linked from the dashboard.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/app.routes.ts

@@ -11,6 +11,7 @@ export const routes: Routes = [
       { path: 'dashboard', loadComponent: () => import('./pages/dashboard/dashboard.page').then(m => m.DashboardPage) },
       { path: 'registratie', loadComponent: () => import('./pages/registration-detail/registration-detail.page').then(m => m.RegistrationDetailPage) },
       { path: 'herregistratie', loadComponent: () => import('./pages/herregistratie/herregistratie.page').then(m => m.HerregistratiePage) },
+      { path: 'concepts', loadComponent: () => import('./pages/concepts/concepts.page').then(m => m.ConceptsPage) },
       { path: '**', redirectTo: 'login' },
     ],
   },