Support Kiwix User library folder as input

Current design revolves around a careful queries to add torrents.

One very handy useful use case might be for an existing kiwix-serve or kiwix desktop/apple user to simply seed his ZIM folder.
The main difference would be that the user doesn't query on the seeder but instead uses Kiwix normally and the seeder would look for ZIMs in the specified folder to match books or titles and then torrents.

The only issue with this would be that ZIMs would get added/removed outside of the lifecycle of Kiwix.
- there's a feature on kiwix-desktop to watch a folder I believe, we'll have to promote that in the seeder/doc
- macOS version does not have such feature. We'll have to open a ticket for it.

Start by reading the seeder's existing query and torrent-adding workflow, then inspect how a configured ZIM folder could be scanned and matched to books or titles. Clarify how additions and removals outside Kiwix's lifecycle should be handled, and review Kiwix desktop folder-watching behavior before defining done.

Written by the indexing model from the issue text.