fix(mobile): improved the visibility of backup cloud icon on lighter images (#15886)

* fix(mobile): improved the visibility of backup cloud icon on lighter images

* refactor(mobile): add 'const' keyword to Offset constructor for improved performance

mobile/lib/widgets/asset_grid/thumbnail_image.dart

@@ -204,6 +204,13 @@ class ThumbnailImage extends ConsumerWidget {
                     storageIcon(asset),
                     color: Colors.white.withOpacity(.8),
                     size: 16,
+                    shadows: [
+                      Shadow(
+                        blurRadius: 5.0,
+                        color: Colors.black.withOpacity(0.6),
+                        offset: const Offset(0.0, 0.0),
+                      ),
+                    ],
                   ),
                 ),
               if (asset.isFavorite)