To test: install, open https://music.youtube.com, play any track, then press the fullscreen button in the player bar (left of the expand button), or press G. C toggles the lyrics panel, Q the queue.
The only network request the extension makes is to lrclib.net, to look up timed lyrics by track title/artist/album/length. It is declared as websiteContent and can be turned off in the toolbar popup ("Synced lyrics"). No background script, no remote code, one permission (storage).
