Fix stale-node crash in UI sound auto-wiring

node_added already fires once per node as it enters the tree (including
every descendant of a newly-added subtree), so the recursive call_deferred
in _on_node_added was redundant and held a raw Node reference across a
frame boundary. If that node got freed before the deferred call fired
(e.g. a container torn down and rebuilt the same frame), the engine's
message queue couldn't marshal the argument and threw "Cannot convert
argument 1 from Object to Object". Wiring synchronously removes the
window entirely.

Also silences the intentional integer division in _duration_text,
matching the same annotation already applied to _elapsed_simulated_text.

scripts/autoload/ui_sound_service.gd

@@ -88,7 +88,8 @@ func _next_available_player() -> AudioStreamPlayer:
 
 
 func _on_node_added(node: Node) -> void:
-	call_deferred("_wire_node", node)
+	if node is Button:
+		_wire_button(node)
 
 
 func _wire_existing_buttons() -> void: