fix: use DTO for bank account

src/main/java/no/ntnu/idi/stud/savingsapp/dto/user/BankAccountResponseDTO.java

+package no.ntnu.idi.stud.savingsapp.dto.user;
+
+import java.math.BigDecimal;
+import lombok.Data;
+
+@Data
+public final class BankAccountResponseDTO {
+  private Long bban;
+  private BigDecimal balance;
+}

src/main/java/no/ntnu/idi/stud/savingsapp/dto/user/UserDTO.java

@@ -16,8 +16,8 @@ public class UserDTO {
   private Timestamp createdAt;
   private String role;
   private String subscriptionLevel;
-  private Long checkingAccountBBAN;
-  private Long savingsAccountBBAN;
+  private BankAccountResponseDTO checkingAccountBBAN;
+  private BankAccountResponseDTO savingsAccountBBAN;
   private PointDTO point;
   private StreakDTO streak;
 }