Add hints for iOS

lib/ui/free_space_page.dart

@@ -159,8 +159,6 @@ class _FreeSpacePageState extends State<FreeSpacePage> {
     final result = await deleteLocalFiles(context, status.localIDs);
     if (result) {
       Navigator.of(context).pop(true);
-    } else {
-      showToast("could not free up space");
     }
   }
 }

lib/utils/delete_file_util.dart

@@ -154,8 +154,12 @@ Future<bool> deleteLocalFiles(
     _logger.info(deletedFiles.length.toString() + " files deleted locally");
     Bus.instance
         .fire(LocalPhotosUpdatedEvent(deletedFiles, type: EventType.deleted));
+    if (Platform.isIOS) {
+      showToast("also empty \"Recently Deleted\" from Settings to claim the freed up space");
+    }
     return true;
   } else {
+    showToast("could not free up space");
     return false;
   }
 }