feat(join): add channel joining flag + style fixes
fix(tracklist): fix tracks continue to play after channel leave
feat(tracklist): implement track move

- movement if tracks is synched via backend

Closes #5
chore(client): restructure files
docs(README): add subdirectory docs
fix(server): fix multi join possible

- add manual leave before repeated join on server
feat(undo): add undo support for actions

fix track move
- playlist sync on move
- move by position to avoid parallel update conflicts
- local move delayed until server sync

update track delete (rename Disco#remove > #delete)

add undo
- server may attach undo message to client update