[Feature Request]: In App Downloads / Download Manager

[MarmadileManteater]

Guidelines

• I have searched the issue tracker for open and closed issues that are similar to the feature request I want to file, without success.
• I have searched the documentation for information that matches the description of the feature request I want to file, without success.
• This issue contains only one feature request.

Problem Description

Changing networks and being on the road can cause video playback to stutter or be inconsistent, but this isn't really the fault of FreeTube. Its just the nature of mobile networks. Videos can currently be downloaded through the web browser, but there is no way to download and mux dash formats. There is also no way to playback a playlist of downloaded videos (say for a roadtrip playlist or podcast).

Proposed Solution

• Bundle ffmpegkit for muxing streams
• Build a download manager view
• Allow a downloads directory to be set within Download Settings
• If a downloads directory is set, override the current download button behaviour
• Display download and ffmpeg progress to user (perhaps with a widget on the watch page or with toast messages)
• Create a database of downloaded video metadata
• Allow for picking of a new playback format called "Downloaded Formats" which will be the default format on any video already downloaded

Alternatives Considered

I've considered attempting to build a download manager which works with ffmpeg wasm, but that is significantly slower on platforms where alternatives are available, and additionally, I ran into problems storing large videos in memory for ffmpeg wasm. Ffmpeg wasm includes functions for downloading files which shouldn't cause the same memory issues I'm running into, but because of youtube's requirements for requesting their streams (range parameter instead of header, with 10mb chunks), ffmpeg wasm's download functions are slow and ineffective.

Issue Labels

new feature

Additional Information

I already started working on this idea, but I haven't touched the branch since before 0.20.0.

This is my WIP branch: feat/download-manager