fix: resolve cart functionality issues with product name and frontend integration

Fixed two critical issues preventing cart from working:

1. Product Name Attribute Error:
   - Product model doesn't have 'name' attribute directly
   - Product name comes from marketplace_product.title relationship
   - Updated all references in cart_service.py:
     * get_cart() - product display in cart
     * add_to_cart() - logging and exception messages
     * update_cart_item() - exception messages
   - Also updated image_url to use marketplace_product.image_link

2. Products Page Not Calling API:
   - products.html had stub addToCart() with TODO comment
   - Implemented actual API call:
     * POST to /api/v1/shop/cart/{sessionId}/items
     * Sends product_id and quantity
     * Updates cartCount on success
     * Shows error toast on failure
     * Proper error handling with try/catch

Testing Results:
- API endpoint now works correctly (tested with curl)
- Cart items are properly saved to database
- Product names display correctly from marketplace_product relationship
- Frontend can successfully add items to cart

Related Models:
- Product has relationship to MarketplaceProduct
- Product.marketplace_product.title contains the product name
- Product.marketplace_product.image_link contains the image URL

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

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

app/services/cart_service.py

@@ -81,11 +81,11 @@ class CartService:
 
             items.append({
                 "product_id": product.id,
-                "product_name": product.name,
+                "product_name": product.marketplace_product.title,
                 "quantity": cart_item.quantity,
                 "price": cart_item.price_at_add,
                 "line_total": line_total,
-                "image_url": product.image_link if hasattr(product, 'image_link') else None,
+                "image_url": product.marketplace_product.image_link if product.marketplace_product else None,
             })
 
             subtotal += line_total
@@ -157,10 +157,10 @@ class CartService:
             raise ProductNotFoundException(product_id=product_id, vendor_id=vendor_id)
 
         logger.info(
-            f"[CART_SERVICE] Product found: {product.name}",
+            f"[CART_SERVICE] Product found: {product.marketplace_product.title}",
             extra={
                 "product_id": product_id,
-                "product_name": product.name,
+                "product_name": product.marketplace_product.title,
                 "available_inventory": product.available_inventory
             }
         )
@@ -195,7 +195,7 @@ class CartService:
                 )
                 raise InsufficientInventoryForCartException(
                     product_id=product_id,
-                    product_name=product.name,
+                    product_name=product.marketplace_product.title,
                     requested=new_quantity,
                     available=product.available_inventory
                 )
@@ -230,7 +230,7 @@ class CartService:
                 )
                 raise InsufficientInventoryForCartException(
                     product_id=product_id,
-                    product_name=product.name,
+                    product_name=product.marketplace_product.title,
                     requested=quantity,
                     available=product.available_inventory
                 )
@@ -319,7 +319,7 @@ class CartService:
         if product.available_inventory < quantity:
             raise InsufficientInventoryForCartException(
                 product_id=product_id,
-                product_name=product.name,
+                product_name=product.marketplace_product.title,
                 requested=quantity,
                 available=product.available_inventory
             )