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_native.dart

@@ -0,0 +1,6 @@
+import 'package:dio/dio.dart';
+
+/// Create a Dio instance for native platforms (mobile, desktop).
+Dio createDio(BaseOptions options) {
+  return Dio(options);
+}