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>

static/shop/img/placeholder.svg

@@ -0,0 +1,6 @@
+<svg width="400" height="400" xmlns="http://www.w3.org/2000/svg">
+  <rect width="400" height="400" fill="#f3f4f6"/>
+  <text x="50%" y="50%" font-family="Arial, sans-serif" font-size="20" fill="#9ca3af" text-anchor="middle" dominant-baseline="middle">
+    No Image Available
+  </text>
+</svg>