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

Lua dev mode functional

clippy

whitelist -> allowlist

Add SPDX info to all files

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.