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/app.config.ts

@@ -1,10 +1,11 @@
 import { ApplicationConfig, LOCALE_ID, provideBrowserGlobalErrorListeners } from '@angular/core';
 import { provideRouter } from '@angular/router';
-import { provideHttpClient } from '@angular/common/http';
+import { provideHttpClient, withInterceptors } from '@angular/common/http';
 import { registerLocaleData } from '@angular/common';
 import localeNl from '@angular/common/locales/nl';
 
 import { routes } from './app.routes';
+import { scenarioInterceptor } from './core/scenario.interceptor';
 
 registerLocaleData(localeNl);
 
@@ -12,7 +13,7 @@ export const appConfig: ApplicationConfig = {
   providers: [
     provideBrowserGlobalErrorListeners(),
     provideRouter(routes),
-    provideHttpClient(),
+    provideHttpClient(withInterceptors([scenarioInterceptor])),
     { provide: LOCALE_ID, useValue: 'nl' },
   ]
 };