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/cart.html

@@ -45,7 +45,7 @@
                 <template x-for="item in items" :key="item.product_id">
                     <div class="cart-item-card">
                         <div class="item-image">
-                            <img :src="item.image_url || '/static/images/placeholder.png'"
+                            <img :src="item.image_url || '/static/shop/img/placeholder.svg'"
                                  :alt="item.name">
                         </div>