Name popups *Popup and give scripts, scenes and classes matching names

Every popup is now called *Popup instead of *Window in its file, root node,
node name, class and variable, and PopupWindow becomes BasePopup. Player is
PlayerRow, SongListObject is SongRow and Character is CharacterCard. Script
files are snake_case and scene files PascalCase, each matching its class or
root node, and LightGrayTheme.tres is light_gray_theme.tres.

qr_popup.gd

@@ -0,0 +1,31 @@
+class_name QRPopup
+extends BasePopup
+
+@onready
+var qr_panel: Panel = $QRPanel
+
+@onready
+var qr_code_rect: QRCodeRect = $QRPanel/QRCodeRect
+
+@onready
+var url_label: Label = $QRPanel/URLLabel
+
+
+func _ready() -> void:
+	qr_code_rect.light_module_color = Color.WHITE
+	qr_code_rect.dark_module_color = Color.BLACK
+	qr_code_rect.quiet_zone_size = 4
+	
+	# Generate QR code for the search URL
+	var url: String = Settings.default_path + "/search"
+	qr_code_rect.data = url
+	url_label.text = qr_code_rect.data
+	GameLog.debug("QR code generated for: ", url)
+
+func open() -> void:
+	super()
+	# Regenerate QR code in case settings changed
+	var url: String = Settings.default_path + "/search"
+	qr_code_rect.data = url
+	url_label.text = qr_code_rect.data
+