fix(auth): enable cookie credentials for web API requests

Configure Dio with BrowserHttpClientAdapter and withCredentials: true
for web platform, allowing session cookies to be sent with XHR requests.
This fixes NPM forward auth not working for API calls.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/core/api/api_client_web.dart

@@ -0,0 +1,12 @@
+import 'package:dio/dio.dart';
+import 'package:dio_web_adapter/dio_web_adapter.dart';
+
+/// Create a Dio instance for web platform with credentials support.
+///
+/// Enables `withCredentials` to send cookies with requests, which is
+/// required for NPM forward auth to work correctly.
+Dio createDio(BaseOptions options) {
+  final dio = Dio(options);
+  dio.httpClientAdapter = BrowserHttpClientAdapter(withCredentials: true);
+  return dio;
+}