fix(mobile-app): 修复 CityPickers locationCode 类型错误

添加空字符串默认值，确保 locationCode 参数类型为 String

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mobile-app/lib/features/authorization/presentation/pages/authorization_apply_page.dart

@@ -250,7 +250,7 @@ class _AuthorizationApplyPageState
               height: 300,
               showType: ShowType.pc, // 只显示省市两级
               barrierDismissible: true,
-              locationCode: tempCityCode ?? tempProvinceCode, // 默认选中保存的位置
+              locationCode: tempCityCode ?? tempProvinceCode ?? '', // 默认选中保存的位置
             );
 
             if (result != null) {