Fixed backend API, again.

Turns out track.getInfo just doesn't feel like serving image data anymore, even though the example response shows it. I love last.fm so much.

Various fixes

Add SPDX info to all files

Unify import style

Move caching to src/cache/; Finalize API parsing fixes

Font and user cache code has been moved to special files, independant from src/config.rs

API parsing changes have been properly tested, and last.fm API JSON is now trace-logged for debugging convenience.

Reliable user info; Font refactor

Hit a few more endpoints to fix missing images, fallback to default album art just in case.

Refactor the font cache into its own file.

Cleanup

lots of stuff. theming basically done, added some logging, frontend basically done.

fix packages, implement caching

API deserialization structs