Fix: Handle designJson as string in checkout API

The designJson field can be either a parsed object or a JSON string
depending on how it was stored. The checkout API was assuming it was
always an object and casting it, which caused 'Cannot read properties
of undefined' error when trying to access .length on front/back arrays.

Now properly parses the string if needed and safely accesses array
properties with optional chaining.

.claude/settings.local.json

@@ -64,7 +64,8 @@
       "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: Calculate effectivePrice in design API endpoints *)"
+      "Bash(git commit -m 'Fix: Calculate effectivePrice in design API endpoints *)",
+      "Bash(git commit -m 'Fix: Handle designJson as string in checkout API *)"
     ]
   }
 }