Investigate PWA with offline track download

[aMytho]

Might be useful to add PWA support. We could let the user download a limited section of their library for offline use.

[aMytho]

This is possible!

We need to enable PWA and request the storage persistence permission.

We also need to abstract track loading. We should probably download the metadata for the files that they want, as well as additional abstractions for metadata access. I'm hoping this won't be too complex. The main goal is offline listening, so if this is too complex we can fallback to just that.