fix: Improve invoice customer data handling and validation

- Make customerInfo fields conditionally required based on customer relationship
- Add admin UI conditional visibility to hide embedded fields when relationship exists
- Fix address field naming inconsistency (postal_code -> postalCode)
- Update types to properly reflect optional customerInfo/billingAddress
- Add validation to ensure either customer relationship or embedded info is provided

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

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

src/types/index.ts

@@ -140,7 +140,7 @@ export interface CustomerRecord {
     country?: string
     line1?: string
     line2?: string
-    postal_code?: string
+    postalCode?: string
     state?: string
   }
   createdAt: string