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/assets/images/placeholder.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 300 300" role="img" aria-label="No image available">
+  <rect width="300" height="300" fill="#f1f1f1"/>
+  <g fill="none" stroke="#c7c7c7" stroke-width="8" stroke-linejoin="round" stroke-linecap="round">
+    <rect x="40" y="60" width="220" height="180" rx="10"/>
+    <circle cx="100" cy="115" r="18"/>
+    <path d="M40 210 L120 140 L170 185 L210 150 L260 200 L260 240 L40 240 Z"/>
+  </g>
+</svg>