Lower Case Bug Fixed on Backend

backend/src/main/java/com/example/backend/anwprj/controller/UserController.java

@@ -51,6 +51,7 @@ public class UserController {
             }
         }
 
+        mail = mail.toLowerCase();
         String sessionID = UserSessionIDHandler.addUser(mail, password);
 
         return new ResponseEntity<>(sessionID, HttpStatus.OK);