fix(storefront): add missing placeholder image asset and onerror fallback

getMainImage() referenced /assets/images/placeholder.svg as the no-image
fallback, but src/assets/images/ never existed - any item with zero
photos rendered a browser broken-image icon instead of a placeholder.
Added the asset.

Also added an (error) handler (onImageError) on every dynamic <img> that
renders a user/admin-supplied URL (product card, cart line item, cart
payment QR code, product gallery main + thumbnails) so a 404'd/broken
image URL swaps to the shared placeholder instead of shipping broken.

src/app/pages/cart/cart.component.ts

@@ -9,7 +9,7 @@ import { EMPTY, interval, of, Subscription } from 'rxjs';
 import { catchError, exhaustMap, take, timeout } from 'rxjs/operators';
 import { DeliverySelectorComponent } from '../../components/delivery-selector/delivery-selector.component';
 import { TelegramLoginComponent } from '../../components/telegram-login/telegram-login.component';
-import { getDiscountedPrice, getMainImage, trackByItemId, getBadgeClass, getTranslatedField } from '../../utils/item.utils';
+import { getDiscountedPrice, getMainImage, trackByItemId, getBadgeClass, getTranslatedField, onImageError } from '../../utils/item.utils';
 import { LangRoutePipe } from '../../pipes/lang-route.pipe';
 import { TranslatePipe } from '../../i18n/translate.pipe';
 import { TranslateService } from '../../i18n/translate.service';
@@ -255,6 +255,7 @@ export class CartComponent implements OnDestroy {
   }
 
   readonly getMainImage = getMainImage;
+  readonly onImageError = onImageError;
   readonly trackByItemId = trackByItemId;
   readonly getDiscountedPrice = getDiscountedPrice;
   readonly getBadgeClass = getBadgeClass;