Redesign Downloading/Caching options for library and podcasts

[GrakovNe]

The current state of the app’s internal download feature is naive:

Users can cache an entire book to internal device storage and later remove it.
Users cannot currently cache podcasts.
Users with self-signed certificates are unable to download any episode to offline-cache

This issue aims to redesign the download feature to include the following capabilities:

Allow users to cache all chapters of a book and remove them later.
Enable users to cache the next X episodes of a podcast, with optional automatic caching of new episodes.
Potential solutions could include adding a new “Download Manager” screen or exploring other approaches.