fix: use proper SVG placeholder image across all shop templates

Issues fixed:
- placeholder.jpg contained SVG content but browsers won't render SVG in .jpg files
- Multiple templates referenced wrong placeholder paths

Changes:
- Rename placeholder.jpg to placeholder.svg with proper SVG file
- Update product.html to use /static/shop/img/placeholder.svg
- Update products.html to use /static/shop/img/placeholder.svg
- Update cart.html to use /static/shop/img/placeholder.svg (was /static/images/)

All shop pages now correctly display placeholder images when products
have no image_link set.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/templates/shop/product.html

@@ -29,7 +29,7 @@
         <div class="product-images">
             <div class="main-image bg-white dark:bg-gray-800 rounded-lg overflow-hidden mb-4">
                 <img
-                    :src="selectedImage || '/static/shop/img/placeholder.jpg'"
+                    :src="selectedImage || '/static/shop/img/placeholder.svg'"
                     :alt="product?.marketplace_product?.title"
                     class="w-full h-auto object-contain"
                     style="max-height: 600px;"
@@ -186,7 +186,7 @@
                 <div class="bg-white dark:bg-gray-800 rounded-lg shadow-sm hover:shadow-md transition-shadow overflow-hidden cursor-pointer">
                     <a :href="`{{ base_url }}shop/products/${related.id}`">
                         <img
-                            :src="related.marketplace_product?.image_link || '/static/shop/img/placeholder.jpg'"
+                            :src="related.marketplace_product?.image_link || '/static/shop/img/placeholder.svg'"
                             :alt="related.marketplace_product?.title"
                             class="w-full h-48 object-cover"
                         >