6. Services and dependency injection

@Injectable({ providedIn: "root" })                // app-wide singleton, tree-shakable
export class OrderService {
  private http = inject(HttpClient);               // inject() function (or constructor injection)
  list() { return this.http.get<Order[]>("/api/orders"); }
}

// Component-level provider: a new instance per component instance
@Component({ selector: "app-wizard", providers: [WizardState], template: "" })
export class Wizard { state = inject(WizardState); }

// Injection tokens for non-class values
export const API_URL = new InjectionToken<string>("API_URL");
// app.config.ts: providers: [{ provide: API_URL, useValue: "https://api.example.com" }]