Full-width layout, page-shell template, httpResource async states, README

- Fix full-bleed header/footer (align-items:stretch + block hosts; centered
  content column via shared --app-content-max).
- New templates/page-shell (back-link + heading + intro + content, narrow mode);
  all pages refactored to compose it.
- Async state management with native httpResource + <app-async> wrapper that
  renders exactly one of loading/empty/error/loaded (impossible states
  unrepresentable); delayed spinner + skeleton atoms for slow/fast connections.
- Scenario interceptor (?scenario=slow|loading|empty|error) to demo every state.
- Storybook: spinner/skeleton/page-shell/async-states stories.
- README rewritten as a guide (atomic design, reuse benefits, state handling).

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

src/app/pages/login/login.page.ts

@@ -1,15 +1,16 @@
 import { Component, inject } from '@angular/core';
 import { Router } from '@angular/router';
-import { PageLayoutComponent } from '../../templates/page-layout/page-layout.component';
+import { PageShellComponent } from '../../templates/page-shell/page-shell.component';
 import { LoginFormComponent } from '../../organisms/login-form/login-form.component';
 
 @Component({
   selector: 'app-login-page',
-  imports: [PageLayoutComponent, LoginFormComponent],
+  imports: [PageShellComponent, LoginFormComponent],
   template: `
-    <app-page-layout>
+    <app-page-shell heading="Inloggen" width="narrow"
+      intro="Log in op uw persoonlijke BIG-register omgeving.">
       <app-login-form (submit)="login()" />
-    </app-page-layout>
+    </app-page-shell>
   `,
 })
 export class LoginPage {