Add a persisted global App Settings switch. Off keeps the existing per-severity
lifetimes (errors remain sticky); on makes every alert non-expiring and closes
it when its body is clicked. The NotificationService supplies the current
setting to each toast request.
StickyNotificationIntegrationTests covers persistence and both request modes.
618 tests green, clean build with 0 warnings.
Reference: Notification.Wpf exposes a CloseOnClick request option:
https://github.com/G-Stas/Notification.Wpf
