Support flat theme

Support ```LOOKANDFEEL = com.formdev.flatlaf.FlatLightLaf``` <img width="1974" height="1659" alt="Image" src="https://github.com/user-attachments/assets/54619646-4070-439c-9afb-a7895838ef0e" /> To "support" it, I added "flatlaf-3.7.jar" to the "lib/custom/" directory, then edited two files. Set: ```LOOKANDFEEL = com.formdev.flatlaf.FlatLightLaf``` In "basexgui". It gives a clean and modern UI. (https://www.formdev.com/flatlaf/) It may be possible to integrate it better to support the themes, but having the ability to select that theme in the setting if you've got the jar in "lib/custom" would be gret. Current issue is that if you open setting, the line get replaced with whatever was selected in the dropdown. So, flatlaf get removed and you've to edit the file again.