Function register_preferences 

pub fn register_preferences(build: impl Fn() -> AnyPiece + 'static)

Declare the app’s preferences piece (docs/windows.md) — once, in root(), ideally before app_menu. Enables the desktop Preferences window (singleton, primary+,), the auto Settings…/Preferences menu item, and open_preferences everywhere (cover fallback where the toolkit cannot open windows). The window titles itself with options.title; use register_preferences_with to localize it or change the size.