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/templates/shop/products.html

@@ -201,10 +201,38 @@ document.addEventListener('alpine:init', () => {
             this.loadProducts();
         },
 
-        addToCart(product) {
+        async addToCart(product) {
             console.log('[SHOP] Adding to cart:', product);
-            this.showToast(`${product.marketplace_product.title} added to cart`, 'success');
-            // TODO: Implement actual cart functionality
+
+            try {
+                const url = `/api/v1/shop/cart/${this.sessionId}/items`;
+                const payload = {
+                    product_id: product.id,
+                    quantity: 1
+                };
+
+                const response = await fetch(url, {
+                    method: 'POST',
+                    headers: {
+                        'Content-Type': 'application/json'
+                    },
+                    body: JSON.stringify(payload)
+                });
+
+                if (response.ok) {
+                    const result = await response.json();
+                    console.log('[SHOP] Add to cart success:', result);
+                    this.cartCount += 1;
+                    this.showToast(`${product.marketplace_product.title} added to cart`, 'success');
+                } else {
+                    const error = await response.json();
+                    console.error('[SHOP] Add to cart error:', error);
+                    this.showToast(error.message || 'Failed to add to cart', 'error');
+                }
+            } catch (error) {
+                console.error('[SHOP] Add to cart exception:', error);
+                this.showToast('Failed to add to cart', 'error');
+            }
         }
     }));
 });