feat: API 인증 시스템 구현 및 로그인 화면 연동

- AuthService, AuthRemoteDataSource 구현
- JWT 토큰 관리 (SecureStorage 사용)
- 로그인 화면 API 연동 및 에러 처리
- freezed 패키지로 Auth 관련 DTO 모델 생성
- 의존성 주입 설정 업데이트

lib/data/models/auth/login_response.g.dart

@@ -0,0 +1,25 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'login_response.dart';
+
+// **************************************************************************
+// JsonSerializableGenerator
+// **************************************************************************
+
+_$LoginResponseImpl _$$LoginResponseImplFromJson(Map<String, dynamic> json) =>
+    _$LoginResponseImpl(
+      accessToken: json['access_token'] as String,
+      refreshToken: json['refresh_token'] as String,
+      tokenType: json['token_type'] as String,
+      expiresIn: (json['expires_in'] as num).toInt(),
+      user: AuthUser.fromJson(json['user'] as Map<String, dynamic>),
+    );
+
+Map<String, dynamic> _$$LoginResponseImplToJson(_$LoginResponseImpl instance) =>
+    <String, dynamic>{
+      'access_token': instance.accessToken,
+      'refresh_token': instance.refreshToken,
+      'token_type': instance.tokenType,
+      'expires_in': instance.expiresIn,
+      'user': instance.user,
+    };