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/components/product-card/product-card.component.ts

@@ -4,7 +4,7 @@ import { RouterLink } from '@angular/router';
 import { Product } from '../../core/products/models/product-domain.model';
 import { LangRoutePipe } from '../../pipes/lang-route.pipe';
 import { TranslatePipe } from '../../i18n/translate.pipe';
-import { cleanDescription, getBadgeClass, getDiscountedPrice, getMainImage } from '../../utils/item.utils';
+import { cleanDescription, getBadgeClass, getDiscountedPrice, getMainImage, onImageError } from '../../utils/item.utils';
 
 const STOCK_LABEL_KEYS: Record<string, string> = {
   high: 'catalog.stockHigh',
@@ -56,6 +56,7 @@ export class ProductCardComponent {
   readonly getDiscountedPrice = getDiscountedPrice;
   readonly getBadgeClass = getBadgeClass;
   readonly cleanDescription = cleanDescription;
+  readonly onImageError = onImageError;
 
   onAddToCart(event: Event): void {
     this.addToCart.emit({ itemID: this.item.itemID, event });