phase3: nullfix.py batch script — bang/late/param-nullable

Automated null-safety fixes driven by dart analyze (no corruption):
- unchecked_use_of_nullable_value: insert '!' on receiver (property/method/[]/op)
- not_initialized field/var: mark 'late'
- missing_default_value_for_parameter: nullable param
Errors: 2374(peak) -> 907

lib/pages/download.dart

@@ -30,7 +30,7 @@ class Download extends StatefulWidget {
 
 class DownloadState extends State<Download> {
   final _scaffoldKey = GlobalKey<ScaffoldState>();
-  Map<String, dynamic> data;
+  late Map<String, dynamic> data;
 
   @override
   Widget build(BuildContext context) {
@@ -79,7 +79,7 @@ class DownloadState extends State<Download> {
     super.initState();
     eventBus.on<OpenDrawer>().listen((event) {
       if (mounted) {
-        _scaffoldKey.currentState.openDrawer();
+        _scaffoldKey.currentState!.openDrawer();
       }
     });
     _loadData();