Add an option to set an "inbox" category for all newly added archives

[Difegue]

Thinking back on https://github.com/Difegue/LANraragi/discussions/479, it might be convenient to add an option for users to define a category as "default" for all newly added archives, whether through Shinobu, the web uploader or URL downloads.

I have kind of a similar setup in place using a "To Read" category and the built-in support in Tsukihi, so it might be a good idea to generalize this: