feat: surface read-only load errors

lib/app/error_message.dart

@@ -0,0 +1,55 @@
+import 'package:dio/dio.dart';
+
+String describeLoadError(Object error) {
+  if (error is DioException) {
+    final statusCode = error.response?.statusCode;
+
+    if (statusCode == 401 || statusCode == 403) {
+      return 'Нет доступа: API key отклонён ($statusCode).';
+    }
+
+    if (_isNetworkFailure(error)) {
+      return 'Backend недоступен: ${error.message ?? error.type.name}.';
+    }
+
+    final responseMessage = _responseMessage(error);
+    if (responseMessage != null) {
+      return 'Backend вернул ошибку $statusCode: $responseMessage';
+    }
+
+    if (statusCode != null) {
+      return 'Backend вернул ошибку $statusCode.';
+    }
+
+    return 'Ошибка запроса: ${error.message ?? error.type.name}.';
+  }
+
+  if (error is FormatException) {
+    return 'Неожиданный ответ backend: ${error.message}';
+  }
+
+  return error.toString();
+}
+
+bool _isNetworkFailure(DioException error) {
+  return switch (error.type) {
+    DioExceptionType.connectionTimeout ||
+    DioExceptionType.sendTimeout ||
+    DioExceptionType.receiveTimeout ||
+    DioExceptionType.connectionError ||
+    DioExceptionType.unknown => true,
+    _ => false,
+  };
+}
+
+String? _responseMessage(DioException error) {
+  final data = error.response?.data;
+  if (data is Map) {
+    final value = data['detail'] ?? data['message'] ?? data['error'];
+    return value?.toString();
+  }
+  if (data is String && data.trim().isNotEmpty) {
+    return data.trim();
+  }
+  return null;
+}