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

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

@@ -43,7 +43,7 @@ export class CartComponent {
 
   readonly closed = output<void>();
 
-  protected readonly resetKey = signal(0);
+  protected readonly quantityOverrides = signal<Record<number, number>>({});
 
   constructor() {
     this.quantityUpdates$.pipe(
@@ -55,12 +55,13 @@ export class CartComponent {
             next: (res) => {
               const msg = res.message || 'Cantidad de producto actualizada.';
               this.toastService.success(msg);
+              this.clearOverride(update.productVariantId);
             },
             error: (err: HttpErrorResponse) => {
               console.error('Error updating cart quantity', err);
               const msg = err.error?.message || 'Error al actualizar la cantidad del producto.';
               this.toastService.danger(msg);
-              this.resetKey.update(k => k + 1);
+              this.clearOverride(update.productVariantId);
             }
           }),
           catchError(() => EMPTY)
@@ -70,10 +71,29 @@ export class CartComponent {
     ).subscribe();
   }
 
+  protected getItemQuantity(item: CartItemMock): number {
+    if (item.productVariantId !== undefined && this.quantityOverrides()[item.productVariantId] !== undefined) {
+      return this.quantityOverrides()[item.productVariantId];
+    }
+    return item.quantity;
+  }
+
+  private clearOverride(productVariantId: number): void {
+    this.quantityOverrides.update((overrides) => {
+      const copy = { ...overrides };
+      delete copy[productVariantId];
+      return copy;
+    });
+  }
+
   protected onItemQuantityChange(index: number, newQuantity: number): void {
     const mockItem = this.items()[index];
     const productVariantId = mockItem?.productVariantId;
     if (productVariantId) {
+      this.quantityOverrides.update((overrides) => ({
+        ...overrides,
+        [productVariantId]: newQuantity
+      }));
       this.quantityUpdates$.next({
         productVariantId,
         quantity: newQuantity
@@ -81,6 +101,10 @@ export class CartComponent {
     } else {
       const item = this.cartService.cart()?.items[index];
       if (item) {
+        this.quantityOverrides.update((overrides) => ({
+          ...overrides,
+          [item.product_variant_id]: newQuantity
+        }));
         this.quantityUpdates$.next({
           productVariantId: item.product_variant_id,
           quantity: newQuantity