loading state and error handling refactor

lib/api/errors/app_exception.dart

@@ -0,0 +1,16 @@
+abstract class AppException implements Exception {
+  final String userMessage;
+  final String? technicalDetails;
+  final bool allowRetry;
+
+  const AppException({
+    required this.userMessage,
+    this.technicalDetails,
+    this.allowRetry = true,
+  });
+
+  @override
+  String toString() => technicalDetails == null
+      ? '$runtimeType: $userMessage'
+      : '$runtimeType: $userMessage ($technicalDetails)';
+}