added devcontainer with all tools for frontent (deno, nextjs) and backend (dart, go)

backend-dart/lib/domain/repositories/user_repository.dart

@@ -0,0 +1,7 @@
+import 'package:backend_dart/domain/entities/user.dart';
+
+abstract class UserRepository {
+  Future<void> create(User user);
+  Future<User?> findByEmail(String email);
+  Future<User?> findById(String id);
+}