Returning [.alert, .sound] from willPresent shows the banner but never
adds the entry to Notification Center — .alert has been a banner-only
shim since macOS 11. On 11+ we need [.banner, .list] for the entry to
stick after the banner auto-dismisses.
Generic OS-notification + window-focus surface. Hand-rolled MethodChannel
+ EventChannel, registered through XPlugin alongside the existing camera
/ video / clipboard plugins. macOS native handler uses
UNUserNotificationCenter with thread-grouping support, NSApp activation
on tap, and NSApplicationDidBecomeActive/DidResignActive for the focus
signal (more reliable than Flutter's AppLifecycleState on macOS).
