Fixed structure of UI, Added some logic to Auth

src/main/java/service/UserService.java

@@ -15,10 +15,7 @@ public class UserService {
 
     public Optional<User> login(String username, String password) {
         try {
-
-
-
-            return Optional.empty();
+            return userRepository.getUser(username, password);
         } catch (Exception e) {
             System.out.println(e.getMessage());
             return Optional.empty();
@@ -27,6 +24,10 @@ public class UserService {
 
     public Optional<User> register(String username, String email, String password) {
         try {
+            if (userRepository.userExists(username)) {
+                return Optional.empty();
+            }
+
             return userRepository.saveUser(username, email, password);
         } catch (Exception e) {
             e.printStackTrace();