Enforcing Username Uniqueness (#906)

* Enforcing Username Uniqueness

Changes in UserService.java:

Added a new method findByUsername to allow searching for usernames regardless of case sensitivity.
Added a new method isUsernameValid to validate the username.
Changes in UserController.java:

Updated the changeUsername method to ensure the new username is valid before changing it.
Updated the editUser method to ensure the new username is unique and valid.
Changes in UserRepository.java:

Added a custom JPQL query to search for usernames regardless of case sensitivity.
Changes in HTML templates (account.html and addUsers.html):

Error messages are displayed if a username is invalid or already exists.

* JPAs auto

src/main/resources/messages_ko_KR.properties

@@ -53,6 +53,7 @@ notAuthenticatedMessage=User not authenticated.
 userNotFoundMessage=사용자를 찾을 수 없습니다.
 incorrectPasswordMessage=현재 비밀번호가 틀립니다.
 usernameExistsMessage=새 사용자명이 이미 존재합니다.
+invalidUsernameMessage=Invalid username, Username must only contain alphabet characters and numbers.
 deleteCurrentUserMessage=Cannot delete currently logged in user.
 deleteUsernameExistsMessage=The username does not exist and cannot be deleted.
 
@@ -145,6 +146,7 @@ adminUserSettings.header=사용자 관리
 adminUserSettings.admin=관리자
 adminUserSettings.user=사용자
 adminUserSettings.addUser=새 사용자 추가
+adminUserSettings.usernameInfo=Username must only contain letters and numbers, no spaces or special characters.
 adminUserSettings.roles=역할
 adminUserSettings.role=역할
 adminUserSettings.actions=동작