simplify: dedupe boilerplate and remove dead code across all layers

lib/widget/async_actions/async_action_controller.dart

@@ -3,6 +3,11 @@ part of '../async_action_button.dart';
 typedef AsyncActionCallback = Future<void> Function();
 typedef AsyncErrorBuilder = String Function(Object error);
 
+/// Shared style for the inline error message the async-button family renders
+/// beneath a control after a caught failure.
+TextStyle _asyncErrorTextStyle(BuildContext context) =>
+    TextStyle(color: Theme.of(context).colorScheme.error, fontSize: 13);
+
 /// Wraps [action] with a try/catch that pops up an [InfoDialog] on failure
 /// (using [errorBuilder] or the default error mapper). Returns `true` on
 /// success, `false` on caught failure.