Full-screen layer-shell overlay containing a game library grid
(XDG Game category, filtered via ApplicationListSearchModel) and
a panel of running tasks built on TaskManager.TasksModel.
Keyboard-navigable with directional focus between the running
tasks row and the game grid. A persistent HUD button lets the
user return after launching a game. Exiting gaming mode requires
an explicit confirmation dialog.
The overlay is a Window with LayerShell.LayerTop so it sits above
running application windows without covering system notifications.
