Import multiple books or folder

Is your feature request related to a problem? Please describe. I'm always frustrated when I have to import books one by one into Foliate. Importing multiple books individually is time-consuming and inconvenient, especially when managing large collections.

Describe the solution you'd like I would like the ability to import multiple books at once, either by selecting multiple files or by selecting an entire folder containing books. This would streamline the importing process and make it much faster for users with large libraries.

Describe alternatives you've considered An alternative solution might be to drag and drop several files into the interface. Another option could be supporting import from zip archives containing multiple books.

Additional context Other eBook apps often support importing multiple files or whole folders, which greatly improves usability for users with large collections. Screenshots or more detailed workflow ideas can be provided if needed.