phase3: bulk null-safety fixes

- validator: (String value) -> (String? value) [30 files]
- ElevatedButton.styleFrom(primary:) -> backgroundColor: [31 files]
- global Key? param fix [129 files]

lib/widgets/general/download_item.dart

@@ -244,7 +244,7 @@ class DownloadItemState extends State<DownloadItem> {
     } else if (downloadUrl != null && downloadUrl.isNotEmpty) {
       return ElevatedButton(
         style: ElevatedButton.styleFrom(
-          primary: Theme.of(context).primaryColor,
+          backgroundColor: Theme.of(context).primaryColor,
           padding: EdgeInsets.all(8)
         ),
         child: Row(