Fix: Calculate effectivePrice in design API endpoints

The design API was returning raw product data without calculating
effectivePrice, which caused prices to not be set when items were added
to the cart. Now uses toProductDTO to properly calculate the effective
price (accounting for sales) before returning design data.

Fixes issue where design reviews showed correct price but cart showed £0.00

.claude/settings.local.json

@@ -63,7 +63,8 @@
       "Bash(git show *)",
       "Bash(git status *)",
       "Bash(git revert *)",
-      "Bash(git commit -m 'Fix price display issues and layout in design review page *)"
+      "Bash(git commit -m 'Fix price display issues and layout in design review page *)",
+      "Bash(git commit -m 'Fix: Calculate effectivePrice in design API endpoints *)"
     ]
   }
 }