Better download/offline play support

[Spatchy]

Hi, I really like Chora and appreciate your work, but unfortunately I'm unable to daily-drive it due to issues with downloading and offline playback.

Currently, downloading a song or playlist dumps it unceremoniously in the default Android music directory. Chora can't find that music unless you then add that directory as a local source and switching between Navidrome and local sources when connection is lost is finicky.

Download management is not ideal for long playlists, with 2 notifications being spammed for each song (a "currently downloading" and then a "download successful" notification), and as far as I can tell, there is no indication in-app that a song is locally downloaded, which allows a song to be downloaded multiple times, resulting in duplicate files.

I would like to see:

1. An in-app download manager for songs and playlists, showing the current download progress
2. An indication on each song if it has been downloaded
3. Deduplication (if a song has already been downloaded i.e. as part of a playlist, it can't be downloaded again)
4. prioritise local files if present, to save data usage (while still scrobbling back to the server)
5. Support seamlessly switching between offline and online playback when connection is lost (only play locally available files in the queue, display network error messages for lyrics, etc.)
6. If a playlist is downloaded, any new songs added to the playlist should be downloaded automatically

I understand that this is asking a lot, but gradually implementing these would massively improve the user experience and make Chora the best mobile Navidrome client imo.