Fix download asset loading indicator position (#2974)

mobile/lib/modules/asset_viewer/views/video_viewer_page.dart

@@ -73,9 +73,13 @@ class VideoViewerPage extends HookConsumerWidget {
           placeholder: placeholder,
         ),
         if (downloadAssetStatus == DownloadAssetStatus.loading)
-          const Center(
+          SizedBox(
+            height: MediaQuery.of(context).size.height,
+            width: MediaQuery.of(context).size.width,
+            child: const Center(
               child: ImmichLoadingIndicator(),
             ),
+          ),
       ],
     );
   }