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/config/security/UserService.java

@@ -176,6 +176,10 @@ public class UserService implements UserServiceInterface {
         return userRepository.findByUsername(username);
     }
 
+    public Optional<User> findByUsernameIgnoreCase(String username) {
+        return userRepository.findByUsernameIgnoreCase(username);
+    }
+
     public void changeUsername(User user, String newUsername) {
         user.setUsername(newUsername);
         userRepository.save(user);
@@ -194,4 +198,8 @@ public class UserService implements UserServiceInterface {
     public boolean isPasswordCorrect(User user, String currentPassword) {
         return passwordEncoder.matches(currentPassword, user.getPassword());
     }
+
+    public boolean isUsernameValid(String username) {
+        return username.matches("[a-zA-Z0-9]+");
+    }
 }