🎨 Use feature based folder structure

lib/auth/captcha.config.dart

@@ -0,0 +1,11 @@
+import 'package:island/core/network.dart';
+import 'package:riverpod_annotation/riverpod_annotation.dart';
+
+part 'captcha.config.g.dart';
+
+@riverpod
+Future<String> captchaUrl(Ref ref) async {
+  final apiClient = ref.watch(apiClientProvider);
+  final baseUrl = await apiClient.get('/config/site');
+  return '$baseUrl/auth/captcha';
+}