Creation of side-menu.

frontend/src/app/core/rest-services/category/category.rest-service.ts

@@ -0,0 +1,15 @@
+import { HttpClient } from '@angular/common/http';
+import { Injectable, inject } from '@angular/core';
+import { lastValueFrom } from 'rxjs';
+import { Category } from './model/category';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class CategoryRestService {
+  private httpClient = inject(HttpClient);
+
+  getCategories(): Promise<Category[]> {
+    return lastValueFrom(this.httpClient.get<Category[]>('/api/categories'));
+  }
+}

frontend/src/app/core/rest-services/category/model/category.ts

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+export interface Category {
+    id: string;
+    name: string;
+    subCategories: Category[];
+}

frontend/src/app/core/service/category.service.ts

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+import { Injectable, inject } from '@angular/core';
+import { CategoryRestService } from '../rest-services/category/category.rest-service';
+import { BehaviorSubject, Observable } from 'rxjs';
+import { Category } from '../rest-services/category/model/category';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class CategoryService {
+  private categoryRestService = inject(CategoryRestService);
+  private categoriesSubject = new BehaviorSubject<Category[]>([]);
+
+  private get categories(): Category[] {
+    return this.categoriesSubject.value;
+  }
+
+  get categories$(): Observable<Category[]> {
+    if (!this.categories?.length) {
+      this.categoryRestService.getCategories()
+      .then(categories => this.categoriesSubject.next(categories))
+      .catch(error => console.error('An error occured while loading categories.', error));
+    }
+    return this.categoriesSubject.asObservable();
+  }
+}