Add AuthService initial cut

lib/services/auth_service.dart

@@ -0,0 +1,31 @@
+import 'package:flutter/foundation.dart';
+import 'package:result_monad/result_monad.dart';
+
+import '../models/exec_error.dart';
+import '../friendica_client.dart';
+
+class AuthService extends ChangeNotifier {
+  FriendicaClient? _friendicaClient;
+
+  Result<FriendicaClient, ExecError> get currentClient {
+    if (_friendicaClient == null) {
+      return Result.error(ExecError(
+        type: ErrorType.authentication,
+        message: 'Not logged in',
+      ));
+    }
+
+    return Result.ok(_friendicaClient!);
+  }
+
+  Result<FriendicaClient, ExecError> updateClient(FriendicaClient newClient) {
+    _friendicaClient = newClient;
+    notifyListeners();
+    return Result.ok(newClient);
+  }
+
+  void clearCredentials() {
+    _friendicaClient = null;
+    notifyListeners();
+  }
+}