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.html

@@ -1,7 +1,7 @@
 <article class="product-card" [class.product-card-compact]="appearance === 'compact'" (mouseenter)="preview.emit(item.itemID)">
   <a [routerLink]="['/product', item.itemID] | langRoute" class="product-link" (click)="onSelected($event)">
     <div class="product-image">
-      <img [src]="getMainImage(item)" [alt]="title || item.name" loading="lazy" decoding="async" width="300" height="300" />
+      <img [src]="getMainImage(item)" [alt]="title || item.name" loading="lazy" decoding="async" width="300" height="300" (error)="onImageError($event)" />
       @if (showDiscountBadge && item.discount > 0) {
         <div class="discount-badge" [attr.aria-label]="'common.discountLabel' | translate:{ value: item.discount }">-{{ item.discount }}%</div>
       }