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/java/stirling/software/SPDF/repository/UserRepository.java

@@ -3,10 +3,14 @@ package stirling.software.SPDF.repository;
 import java.util.Optional;
 
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.repository.query.Param;
 
 import stirling.software.SPDF.model.User;
 
 public interface UserRepository extends JpaRepository<User, String> {
+    Optional<User> findByUsernameIgnoreCase(String username);
+
     Optional<User> findByUsername(String username);
 
     User findByApiKey(String apiKey);