main/ holds the main window and top bar, quiz/ the quiz and its parts,
player/ the music player, popups/ every popup and what they share, data/ the
songs, players, playlist and settings, server/ the MusicServer client,
common/ the list row and GameLog, and themes/ the themes and style boxes.
MusicListPanel, PlayersPanel (was Players) and InspirationList (was
ScrollContainer) each get their own scene and script. They report what
happens through signals, and the quiz wires them to the playlist, the
music player, the log and the popups. Every node keeps its place on screen.
Screenshots of every UI state are pixel-identical to before, the layout
test still matches the original baseline, and each part is tested on its
own as well as through the quiz.