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/utils/http_util.dart

@@ -35,8 +35,8 @@ class HttpUtil {
     'Http-Contact-Authorization': '',
     'Http-Device-Type': Utils.getOs(checkWeb: true),
     'Http-Api-Branch': 'flutter',
-    'Http-Language-Code': store.state.locale.languageCode,
-    'Http-Country-Code': store.state.locale.countryCode ?? '',
+    'Http-Language-Code': store.state.locale!.languageCode,
+    'Http-Country-Code': store.state.locale!.countryCode ?? '',
   };
 
   static Future<dynamic> httpGet(String url,