* 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
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