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/widgets/mobile/mobile_blog.dart

@@ -28,7 +28,7 @@ class MobileBlog extends StatefulWidget {
 }
 
 class MobileBlogState extends State<MobileBlog> {
-  List<Blog> blogs;
+  late List<Blog> blogs;
 
   int _page = 1;
   int _pageCount = 1;