Add follow and unfollow for live and offline channels.

Long-press actions use Helix for follow status and web GQL mutations via the linked session.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/apis/base_api_client.dart

@@ -44,6 +44,13 @@ class UnauthorizedException extends ApiException {
     : super(message, 401, DioExceptionType.badResponse);
 }
 
+/// Twitch GQL rejected the request for a missing/invalid Client-Integrity token.
+class IntegrityCheckException extends ApiException {
+  const IntegrityCheckException([
+    String message = 'Twitch integrity check failed',
+  ]) : super(message);
+}
+
 /// Base API client to eliminate code duplication across API services
 abstract class BaseApiClient {
   final Dio _dio;