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/templates/account.html

@@ -30,6 +30,9 @@
               <div th:if="${param.messageType[0] == 'usernameExists'}" class="alert alert-danger">
                 <span th:text="#{usernameExistsMessage}">Default message if not found</span>
               </div>
+              <div th:if="${param.messageType[0] == 'invalidUsername'}" class="alert alert-danger">
+                <span th:text="#{invalidUsernameMessage}">Default message if not found</span>
+              </div>
               </th:block>
               <!-- At the top of the user settings -->
               <h3 class="text-center"><span th:text="#{welcome} + ' ' + ${username}">User</span>!</h3>