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/product_search.dart

@@ -142,7 +142,7 @@ class ProductSearchState extends State<ProductSearch> {
     );
     if (result is DioError) {
       if (result.response != null) {
-        throw RuntimeError(result.response.data);
+        throw RuntimeError(result.response!.data);
       } else {
         throw RuntimeError(result.message);
       }