fix(mobile): Add fade-in to asset viewer transition (#23692)

Add fade-in animation

mobile/lib/routing/router.dart

@@ -313,6 +313,7 @@ class AppRouter extends RootStackRouter {
           settings: page,
           pageBuilder: (_, __, ___) => child,
           opaque: false,
+          transitionsBuilder: TransitionsBuilders.fadeIn,
         ),
       ),
     ),