feat(cart): implement optimistic quantity updates with rollback on error

src/app/shared/components/cart/cart.component.html

@@ -10,7 +10,7 @@
   </header>
 
   <div class="d-grid gap-2 flex-grow-1 overflow-y-auto cart-items-container">
-    @for (item of items(); track item.product + item.discountedPrice + resetKey(); let idx = $index) {
+    @for (item of items(); track item.productVariantId || item.product + item.discountedPrice; let idx = $index) {
       <app-cart-item
         [imageUrl]="item.imageUrl"
         [product]="item.product"
@@ -18,7 +18,7 @@
         [discountedPrice]="item.discountedPrice"
         [discountPercentage]="item.discountPercentage"
         [attributes]="item.attributes"
-        [quantity]="item.quantity"
+        [quantity]="getItemQuantity(item)"
         (quantityChange)="onItemQuantityChange(idx, $event)"
         (remove)="onItemRemove(idx)"
       />