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/desktop/desktop_blog.dart

@@ -34,7 +34,7 @@ class DesktopBlog extends StatefulWidget {
 }
 
 class DesktopBlogState extends State<DesktopBlog> {
-  List<Blog> blogs;
+  late List<Blog> blogs;
 
   double division = 2;