add validation

src/main/java/com/faf223/expensetrackerfaf/dto/UserCreationDTO.java

@@ -1,15 +1,28 @@
 package com.faf223.expensetrackerfaf.dto;
 
+import jakarta.validation.constraints.Email;
+import jakarta.validation.constraints.NotEmpty;
+import jakarta.validation.constraints.NotNull;
 import lombok.AllArgsConstructor;
 import lombok.Data;
 
 @Data
 @AllArgsConstructor
 public class UserCreationDTO {
-  
+    @NotNull(message = "First name must not be null")
+    @NotEmpty(message = "First name must not be empty")
     private String firstname;
+    @NotNull(message = "Last name must not be null")
+    @NotEmpty(message = "Last name must not be empty")
     private String lastname;
+    @NotNull(message = "Username must not be null")
+    @NotEmpty(message = "Username must not be empty")
     private String username;
+    @NotNull(message = "Email must not be null")
+    @NotEmpty(message = "Email must not be empty")
+    @Email(message = "Email must be valid")
     private String email;
+    @NotNull(message = "Password must not be null")
+    @NotEmpty(message = "Password must not be empty")
     private String password;
 }