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.html

@@ -29,7 +29,7 @@
                (touchstart)="onSwipeStart(item.itemID, $event)">
             <div class="cart-item">
               <a [routerLink]="['/product', item.itemID] | langRoute" class="item-image">
-                <img [src]="getMainImage(item)" [alt]="itemName(item)" loading="lazy" />
+                <img [src]="getMainImage(item)" [alt]="itemName(item)" loading="lazy" (error)="onImageError($event)" />
               </a>
               
               <div class="item-info">
@@ -216,7 +216,7 @@
           @if (qrCodeUrl()) {
             <div class="qr-section">
               <div class="qr-wrapper">
-                <img [src]="qrCodeUrl()" [attr.alt]="'cart.paymentQrAlt' | translate" class="qr-code" />
+                <img [src]="qrCodeUrl()" [attr.alt]="'cart.paymentQrAlt' | translate" class="qr-code" (error)="onImageError($event)" />
                 <div class="scan-line"></div>
               </div>
             </div>