add validator for password in userManagement

[Brano5]

This pull request updates the user management form validation logic in UserManagement.razor to improve password handling and input validation. The most notable changes are the introduction of custom validation for password fields and the removal of the minimum password length requirement.

Validation improvements:

• The InputModel class now implements IValidatableObject, allowing for custom validation logic.
• Custom validation ensures that the "Confirm new password" field is only required and compared when a new password is provided, giving more precise feedback to users.

Password requirements adjustment:

• The minimum length restriction for the "New password" field has been removed; only a maximum length of 100 characters is now enforced.
• The [Compare] attribute for confirming the new password has been removed in favor of the new custom validation logic.