Add 'New Update' notification flow

Introduce a small update-notification feature: add AppEvents.HasUpdateNotified event and have TaskManager publish it when update messages exist. Add localized resource key (menuNewUpdate) and expose it in the ResUI designer; update resource files for several languages. In the UI, add a New Update button in MainWindow.xaml, wire its click to the existing check-update handler, bind its visibility to a new BlNewUpdate property on MainWindowViewModel, and subscribe the viewmodel to the new event. Also reset the notification flag after showing the Check Update dialog.