app: [iOS] fix focus event for iOS 13.0+ with backward compatibility

UIScene notifications are the correct way to track window focus on
iOS 13.0+, because the Key Window API is scene-level on iOS 13.0+,
but we need to maintain support for iOS 12 and earlier.

Implementation strategy:
- iOS 13.0+: Use UIScene notifications (DidActivate/WillDeactivate)
- iOS 12 and earlier: Keep existing UIWindow notifications as fallback
- Add runtime version checks to select the appropriate API

Key changes in os_ios.m:
- Add @available(iOS 13.0, *) checks
- Register both notification types with conditional logic
- Ensure proper cleanup of observers when moving between windows

See: https://developer.apple.com/documentation/uikit/uiscene?language=objc

Fixes: https://lists.sr.ht/~eliasnaur/gio/%3CCAMAFT9Uyh_JWrkQQt+AmekJWFBqhZPsP_3ZxC1fUNB+=VGGorw@mail.gmail.com%3E
Signed-off-by: CoyAce <akeycoy@gmail.com>
Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/os.go

@@ -47,7 +47,7 @@ type Config struct {
 	Decorated bool
 	// TopMost windows render above all other non-top-most windows.
 	TopMost bool
-	// Focused reports whether has the keyboard focus.
+	// Focused reports whether the window is focused.
 	Focused bool
 	// decoHeight is the height of the fallback decoration for platforms such
 	// as Wayland that may need fallback client-side decorations.