feat: implement MusicManager singleton and integrate background music into lobby scene

scripts/managers/music_manager.gd

@@ -48,7 +48,7 @@ func start_music():
 		"Stop n Go":
 			track_path = "res://assets/sounds/stop_n_go.wav"
 		"Freemode", "Tekton Doors", _:
-			track_path = "res://assets/sounds/freemode.wav"
+			track_path = "res://assets/sounds/level_bridge.wav"
 	
 	play_track(track_path)
 
@@ -104,4 +104,5 @@ func _on_music_finished():
 	print("[MusicManager] Track finished: ", current_track)
 	# Fallback looping if for some reason the native loop didn't work
 	if current_track != "":
-		print("[MusicManag
+		print("[MusicManager] Restarting track (manual loop fallback): ", current_track)
+		music_player.play()