Add Basque (Euskara) language support for the web interface

Hello,

I would like to request the addition of the Basque (Euskara) language for the Docspell web interface.

Currently, only English, German (Deutsch), and French are available. Adding Basque would be very valuable for users in the Basque Country and surrounding regions, and it could increase adoption of Docspell.

The request is only for the UI translation, not the document processing (NLP).

I (and possibly other contributors) could help with the translation work if needed.

Thank you very much for considering this feature!