fix(mobile): mounted check in ThumbnailTile hero flight listener (#28451)

When the user pops back from the asset viewer mid-flight, the hero
animation can fire its status listener after _ThumbnailTileState has
been disposed. setState then throws a null check on State._element.

Guard the listener with `if (!mounted) return;` — same pattern as
#28300 in the album sync action.

mobile/lib/presentation/widgets/images/thumbnail_tile.widget.dart

@@ -120,6 +120,9 @@ class _ThumbnailTileState extends ConsumerState<ThumbnailTile> {
                     },
                     flightShuttleBuilder: (context, animation, direction, from, to) {
                       void animationStatusListener(AnimationStatus status) {
+                        if (!mounted) {
+                          return;
+                        }
                         final heroInFlight = status == AnimationStatus.forward || status == AnimationStatus.reverse;
                         if (_hideIndicators != heroInFlight) {
                           setState(() => _hideIndicators = heroInFlight);