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.

lib/widget/async_actions/async_dialog_action.dart

@@ -44,10 +44,7 @@ class _AsyncDialogActionState extends State<AsyncDialogAction> {
               child: Text(
                 err,
                 textAlign: TextAlign.center,
-                style: TextStyle(
-                  color: Theme.of(context).colorScheme.error,
-                  fontSize: 13,
-                ),
+                style: _asyncErrorTextStyle(context),
               ),
             ),
           Row(

lib/widget/async_actions/async_list_tile.dart

@@ -76,13 +76,7 @@ class _AsyncListTileState extends State<AsyncListTile> {
           if (err != null)
             Padding(
               padding: const EdgeInsets.only(left: 16, right: 16, bottom: 8),
-              child: Text(
-                err,
-                style: TextStyle(
-                  color: Theme.of(context).colorScheme.error,
-                  fontSize: 13,
-                ),
-              ),
+              child: Text(err, style: _asyncErrorTextStyle(context)),
             ),
         ],
       );

lib/widget/async_actions/async_mixin.dart

@@ -108,10 +108,7 @@ class _InlineErrorWrapper extends StatelessWidget {
               Text(
                 err,
                 textAlign: TextAlign.center,
-                style: TextStyle(
-                  color: Theme.of(context).colorScheme.error,
-                  fontSize: 13,
-                ),
+                style: _asyncErrorTextStyle(context),
               ),
             ],
           ],