feat: API 통합을 위한 기초 인프라 구축

- 네트워크 레이어 구현 (Dio 기반 ApiClient)
- 환경별 설정 관리 시스템 구축
- 의존성 주입 설정 (GetIt)
- API 엔드포인트 상수 정의
- 인터셉터 구현 (Auth, Error, Logging)
- 프로젝트 아키텍처 개선 (core, data, di 디렉토리 구조)
- API 통합 계획서 및 요구사항 문서 작성
- 필요 패키지 추가 (dio, flutter_secure_storage, get_it 등)

lib/core/errors/failures.dart

@@ -0,0 +1,117 @@
+import 'package:dartz/dartz.dart';
+
+/// 실패 처리를 위한 추상 클래스
+abstract class Failure {
+  final String message;
+  final String? code;
+  
+  const Failure({
+    required this.message,
+    this.code,
+  });
+  
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is Failure &&
+          runtimeType == other.runtimeType &&
+          message == other.message &&
+          code == other.code;
+  
+  @override
+  int get hashCode => message.hashCode ^ code.hashCode;
+}
+
+/// 서버 실패
+class ServerFailure extends Failure {
+  final int? statusCode;
+  final Map<String, dynamic>? errors;
+  
+  const ServerFailure({
+    required String message,
+    String? code,
+    this.statusCode,
+    this.errors,
+  }) : super(message: message, code: code);
+}
+
+/// 캐시 실패
+class CacheFailure extends Failure {
+  const CacheFailure({
+    required String message,
+    String? code,
+  }) : super(message: message, code: code);
+}
+
+/// 네트워크 실패
+class NetworkFailure extends Failure {
+  const NetworkFailure({
+    required String message,
+    String? code,
+  }) : super(message: message, code: code);
+}
+
+/// 인증 실패
+class AuthenticationFailure extends Failure {
+  const AuthenticationFailure({
+    required String message,
+    String? code,
+  }) : super(message: message, code: code);
+}
+
+/// 권한 실패
+class AuthorizationFailure extends Failure {
+  const AuthorizationFailure({
+    required String message,
+    String? code,
+  }) : super(message: message, code: code);
+}
+
+/// 유효성 검사 실패
+class ValidationFailure extends Failure {
+  final Map<String, List<String>>? fieldErrors;
+  
+  const ValidationFailure({
+    required String message,
+    String? code,
+    this.fieldErrors,
+  }) : super(message: message, code: code);
+}
+
+/// 리소스 찾을 수 없음 실패
+class NotFoundFailure extends Failure {
+  final String? resourceType;
+  final String? resourceId;
+  
+  const NotFoundFailure({
+    required String message,
+    String? code,
+    this.resourceType,
+    this.resourceId,
+  }) : super(message: message, code: code);
+}
+
+/// 중복 리소스 실패
+class DuplicateFailure extends Failure {
+  final String? field;
+  final String? value;
+  
+  const DuplicateFailure({
+    required String message,
+    String? code,
+    this.field,
+    this.value,
+  }) : super(message: message, code: code);
+}
+
+/// 비즈니스 로직 실패
+class BusinessFailure extends Failure {
+  const BusinessFailure({
+    required String message,
+    String? code,
+  }) : super(message: message, code: code);
+}
+
+/// 타입 정의
+typedef FutureEither<T> = Future<Either<Failure, T>>;
+typedef FutureVoid = FutureEither<void>;