Split the main window into TopBar and Quiz scenes

The quiz (song, playlist, players, music player and the popups opened from
it) moves into Quiz.tscn and the top panel into TopBar.tscn. The main window
keeps only the top bar, the quiz and the popups opened from the top bar, so
other views can be added next to the quiz later. Every node keeps its place
on screen.

Screenshots of every UI state before and after are pixel-identical, and a
layout test checks each node against a baseline taken before the split.

tests/integration/test_layout_unchanged.gd.uid

@@ -0,0 +1 @@
+uid://b2077235y1g0f