Add Spanish (español) language support for the web interface

Hello,

I would like to request the addition of the Spanish language for the Docspell web interface.

Currently, only English, German (Deutsch), and French are available. Spanish would be a very useful addition, since it is one of the most widely spoken languages in the world and could help more users adopt Docspell.

The request is only for the UI translation, not the document processing (NLP), since Spanish is already supported there.

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

Thank you very much for considering this feature!