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_hi_IN.properties

@@ -53,6 +53,7 @@ notAuthenticatedMessage=उपयोगकर्ता प्रमाणित
 userNotFoundMessage=उपयोगकर्ता नहीं मिला।
 incorrectPasswordMessage=वर्तमान पासवर्ड गलत है।
 usernameExistsMessage=नया उपयोगकर्ता नाम पहले से मौजूद है।
+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=व्यवस्थापक उपयोगकर्
 adminUserSettings.admin=व्यवस्थापक
 adminUserSettings.user=उपयोगकर्ता
 adminUserSettings.addUser=नया उपयोगकर्ता जोड़ें
+adminUserSettings.usernameInfo=Username must only contain letters and numbers, no spaces or special characters.
 adminUserSettings.roles=रोल्स
 adminUserSettings.role=रोल
 adminUserSettings.actions=क्रियाएँ