feat: full username instead of username

[01001110J]

Description

This PR updates the header to display the user's full name instead of their username. If the user does not have a full name configured, the username will still be displayed as the default value.

This PR resolves PADV-2063

Change log

• Updated the header to display the full name instead of the username.
• If the full name is not available, the username remains as the fallback.

Visual results

No visual changes.

How to test

1. Start the application.
2. Check the header to confirm that the user's full name is displayed.
3. If the user does not have a full name set, the username should still be displayed.