feat: update cart service and components to handle productVariantId, improve item removal and addition feedback

src/app/features/store/pages/product-detail-page/product-detail-page.component.spec.ts

@@ -59,7 +59,7 @@ describe('ProductDetailPageComponent', () => {
       navigate: vi.fn()
     };
     cartServiceStub = {
-      addItem: vi.fn().mockReturnValue(of({}))
+      addItem: vi.fn().mockReturnValue(of({ message: 'Producto agregado al carrito' }))
     };
     toastServiceStub = {
       success: vi.fn(),
@@ -277,6 +277,8 @@ describe('ProductDetailPageComponent', () => {
   it('keeps quantity at a minimum of one and increments locally', async () => {
     await configureTestingModule();
     const fixture = TestBed.createComponent(ProductDetailPageComponent);
+    fixture.detectChanges();
+
     fixture.componentInstance['selectedVariant'].set({
       variant_id: 1,
       cantidad_maxima: 10,

src/app/features/store/pages/product-detail-page/product-detail-page.component.ts

@@ -230,8 +230,9 @@ export class ProductDetailPageComponent implements OnInit, OnDestroy {
 
     this.addingToCart.set(true);
     this.cartService.addItem(variant.variant_id, this.quantity()).subscribe({
-      next: () => {
-        this.toastService.success('Producto agregado al carrito');
+      next: (res) => {
+        const msg = res.message || 'Producto agregado al carrito';
+        this.toastService.success(msg);
         this.addingToCart.set(false);
       },
       error: (err: HttpErrorResponse) => {