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.dart

@@ -3,23 +3,26 @@ import 'package:riverpod_annotation/riverpod_annotation.dart';
 import 'package:tatlock_ui/core/api/api_interceptors.dart';
 import 'package:tatlock_ui/core/config/app_config.dart';
 
+import 'api_client_native.dart' if (dart.library.html) 'api_client_web.dart'
+    as platform;
+
 part 'api_client.g.dart';
 
 /// Provides the Dio instance for Core API.
 @riverpod
 Dio coreApiClient(Ref ref) {
-  final dio = Dio(
-    BaseOptions(
-      baseUrl: AppConfig.coreApiUrl,
-      connectTimeout: const Duration(seconds: 10),
-      receiveTimeout: const Duration(seconds: 30),
-      headers: {
-        'Content-Type': 'application/json',
-        'Accept': 'application/json',
-      },
-    ),
+  final options = BaseOptions(
+    baseUrl: AppConfig.coreApiUrl,
+    connectTimeout: const Duration(seconds: 10),
+    receiveTimeout: const Duration(seconds: 30),
+    headers: {
+      'Content-Type': 'application/json',
+      'Accept': 'application/json',
+    },
   );
 
+  final dio = platform.createDio(options);
+
   dio.interceptors.addAll([
     AuthInterceptor(ref),
     LoggingInterceptor(),
@@ -32,18 +35,18 @@ Dio coreApiClient(Ref ref) {
 /// Provides the Dio instance for Tatlock API.
 @riverpod
 Dio tatlockApiClient(Ref ref) {
-  final dio = Dio(
-    BaseOptions(
-      baseUrl: AppConfig.tatlockApiUrl,
-      connectTimeout: const Duration(seconds: 10),
-      receiveTimeout: const Duration(minutes: 5), // Longer for LLM responses
-      headers: {
-        'Content-Type': 'application/json',
-        'Accept': 'application/json',
-      },
-    ),
+  final options = BaseOptions(
+    baseUrl: AppConfig.tatlockApiUrl,
+    connectTimeout: const Duration(seconds: 10),
+    receiveTimeout: const Duration(minutes: 5), // Longer for LLM responses
+    headers: {
+      'Content-Type': 'application/json',
+      'Accept': 'application/json',
+    },
   );
 
+  final dio = platform.createDio(options);
+
   dio.interceptors.addAll([
     AuthInterceptor(ref),
     LoggingInterceptor(),

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);
+}

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;
+}