feat: 회사 관리 API 연동 구현

- CompanyService 및 RemoteDataSource 구현
- Company, Branch DTO 모델 생성 (Freezed)
- 의존성 주입 컨테이너 업데이트
- 회사 등록/수정 폼에 API 연동 로직 적용
- API 통합 계획 문서 업데이트

lib/di/injection_container.dart

@@ -6,9 +6,11 @@ import '../data/datasources/remote/api_client.dart';
 import '../data/datasources/remote/auth_remote_datasource.dart';
 import '../data/datasources/remote/dashboard_remote_datasource.dart';
 import '../data/datasources/remote/equipment_remote_datasource.dart';
+import '../data/datasources/remote/company_remote_datasource.dart';
 import '../services/auth_service.dart';
 import '../services/dashboard_service.dart';
 import '../services/equipment_service.dart';
+import '../services/company_service.dart';
 
 /// GetIt 인스턴스
 final getIt = GetIt.instance;
@@ -35,6 +37,9 @@ Future<void> setupDependencies() async {
   getIt.registerLazySingleton<EquipmentRemoteDataSource>(
     () => EquipmentRemoteDataSourceImpl(),
   );
+  getIt.registerLazySingleton<CompanyRemoteDataSource>(
+    () => CompanyRemoteDataSourceImpl(),
+  );
   
   // 서비스
   getIt.registerLazySingleton<AuthService>(
@@ -46,6 +51,9 @@ Future<void> setupDependencies() async {
   getIt.registerLazySingleton<EquipmentService>(
     () => EquipmentService(),
   );
+  getIt.registerLazySingleton<CompanyService>(
+    () => CompanyService(),
+  );
   
   // 리포지토리
   // TODO: Repositories will be registered here