feat: surface read-only load errors

lib/app/load_state.dart

@@ -0,0 +1,34 @@
+enum LoadStatus { idle, loading, data, empty, error }
+
+class LoadState<T> {
+  final LoadStatus status;
+  final T data;
+  final String? errorMessage;
+
+  const LoadState.idle(this.data)
+    : status = LoadStatus.idle,
+      errorMessage = null;
+
+  const LoadState.loading(this.data)
+    : status = LoadStatus.loading,
+      errorMessage = null;
+
+  const LoadState.data(this.data)
+    : status = LoadStatus.data,
+      errorMessage = null;
+
+  const LoadState.empty(this.data)
+    : status = LoadStatus.empty,
+      errorMessage = null;
+
+  const LoadState.error(this.data, this.errorMessage)
+    : status = LoadStatus.error;
+
+  bool get isIdle => status == LoadStatus.idle;
+
+  bool get isLoading => status == LoadStatus.loading;
+
+  bool get hasError => status == LoadStatus.error;
+
+  bool get isEmpty => status == LoadStatus.empty;
+}