fix(public): do not expose coupon code after preregistration

Codes are still created and stored for Mailwizz; JSON response omits
coupon_code. Thank-you UI explains email delivery instead of showing code.

Made-with: Cursor

app/Http/Controllers/PublicPageController.php

@@ -71,7 +71,6 @@ class PublicPageController extends Controller
         return response()->json([
             'success' => true,
             'message' => $publicPage->thank_you_message ?? __('Thank you for registering!'),
-            'coupon_code' => $subscriber->fresh()?->coupon_code,
         ]);
     }