Split now playing, the quiz buttons and song loading out of the quiz

NowPlaying, QuizToolbar and SongControls become their own scenes, the auto
repeat switch moves into the music player scene, and downloading songs moves
into a SongLoader node. The quiz only wires the parts together. 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 new part has its own tests.

MusicPlayer.gd

@@ -1,14 +1,16 @@
 class_name MusicPlayer
-extends PanelContainer
+extends Control
+## The bar that plays the songs, with play, restart, progress and volume, and the
+## auto repeat switch under it.
 
 # LOC 20/1-24: 136
 # LOC 12/6-24: 170
 
 @onready
-var play_button: TextureButton = $MusicPlayerContainer/PlayTextureButton
+var play_button: TextureButton = $PlayerBar/MusicPlayerContainer/PlayTextureButton
 
 @onready
-var restart_button: TextureButton = $MusicPlayerContainer/RestartTextureButton
+var restart_button: TextureButton = $PlayerBar/MusicPlayerContainer/RestartTextureButton
 
 @onready
 var audio_player: AudioStreamPlayer = $AudioStreamPlayer
@@ -20,13 +22,16 @@ var sound_player: AudioStreamPlayer = $AudioSoundPlayer
 var sound_effect_player: AudioStreamPlayer = $AudioSoundEffectPlayer
 
 @onready
-var progress_slider: HSlider = $MusicPlayerContainer/MusicPlayerSlider
+var progress_slider: HSlider = $PlayerBar/MusicPlayerContainer/MusicPlayerSlider
 
 @onready
-var volume_slider: HSlider = $MusicPlayerContainer/VolumeSlider
+var volume_slider: HSlider = $PlayerBar/MusicPlayerContainer/VolumeSlider
 
 @onready
-var music_time_label: Label = $MusicPlayerContainer/MusicTimeLabel
+var music_time_label: Label = $PlayerBar/MusicPlayerContainer/MusicTimeLabel
+
+@onready
+var auto_repeat_button: CheckButton = $RepeatSongCheckButton
 
 var play_icon: Texture = preload("res://icons/play_icon_light.svg")
 var pause_icon: Texture = preload("res://icons/pause_icon_light.svg")
@@ -49,6 +54,10 @@ func _ready() -> void:
 	progress_slider.drag_ended.connect(_on_drag_ended)
 	
 	audio_player.finished.connect(_on_finished)
+	auto_repeat_button.pressed.connect(_on_auto_repeat_pressed)
+
+func _on_auto_repeat_pressed() -> void:
+	Settings.auto_repeat_song = !Settings.auto_repeat_song
 
 func _process(_delta: float) -> void:
 	if audio_player.has_stream_playback() && !is_changing && !audio_player.stream_paused: