applyCode method
validateCode then setReferralCode if valid; sets applied.
Implementation
Future<CodeValidationResult> applyCode(String code,
{String? appUserId,}) async {
final validation = await validateCode(code);
if (!validation.valid) return validation.copyWith(applied: false);
final set = await setReferralCode(code, appUserId: appUserId);
return validation.copyWith(applied: set.success);
}