DankSlideout always held WlrKeyboardFocus.OnDemand regardless of modals,
causing Notepad to steal focus back when Launcher was opened.
ModalManager: clone currentModalsByScreen on mutation so QML bindings
re-evaluate when modals open/close.
DankSlideout: set keyboardFocus to None while a modal is active on the
same screen, letting the modal (launcher) claim focus exclusively.