Offline Reading using Client Device

[calmacleod]

Is your feature request related to a problem? Please describe.
Often times, users will find themselves without an internet connection (like being on a plane) or without a connection to the server leaving them unable to read any of their existing files.

Describe the solution you'd like
Series/Chapters should be allowed to be downloaded and saved locally to a client device which can then be used when there is no connection to a server. Optimistically, any reading progress from an offline session should then sync back to the server when connection is restored.

Describe alternatives you've considered
In the worst case, users could retrieve files manually once they're downloaded to the server and use a document viewer or something similar, however this is a non-optimal solution as it defeats the purpose of a dedicated reader.

Additional context
This feature has been brought up before in #58 and was closed due to requiring a fundamental change. However, from an outsiders perspective it seems all this would require is having a set location on a device series can be stored and using that as a source when no server is directly available.

I don't have any experience with Flutter but would be interested in looking into contributing if it's something that can feasibly be added.

[marimo-nekomimi]

This can probably be achieved by using the Suwayomi extension from Mihon.
It may not be a perfect solution, but please try if possible.
(Of course, this assumes you're an Android user.)