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.
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