feat(core): let AppBanner disable its action instead of hiding it

Refs #1

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

app/lib/core/widgets/app_banner.dart

@@ -17,6 +17,7 @@ class AppBanner extends StatelessWidget {
     super.key,
     this.actionLabel,
     this.onAction,
+    this.actionEnabled = true,
   }) : assert(
          (actionLabel == null) == (onAction == null),
          'actionLabel and onAction belong together',
@@ -28,6 +29,11 @@ class AppBanner extends StatelessWidget {
   final String? actionLabel;
   final VoidCallback? onAction;
 
+  /// `false` keeps the action visible and in the focus order but disabled
+  /// (e.g. while a request runs). Screen readers still find it and hear that
+  /// it is unavailable, instead of the control vanishing under their focus.
+  final bool actionEnabled;
+
   @override
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
@@ -81,7 +87,7 @@ class AppBanner extends StatelessWidget {
               Align(
                 alignment: AlignmentDirectional.centerEnd,
                 child: TextButton(
-                  onPressed: onAction,
+                  onPressed: actionEnabled ? onAction : null,
                   style: TextButton.styleFrom(foregroundColor: foreground),
                   child: Text(actionLabel!),
                 ),

app/test/core/widgets/app_banner_test.dart

@@ -1,3 +1,5 @@
+import 'dart:ui' show Tristate;
+
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:shuffle/core/widgets/app_banner.dart';
@@ -46,6 +48,39 @@ void main() {
       handle.dispose();
     });
 
+    testWidgets('a disabled action stays visible and announced as disabled', (
+      tester,
+    ) async {
+      final handle = tester.ensureSemantics();
+      var calls = 0;
+      await pumpWidgetUnderTest(
+        tester,
+        AppBanner(
+          type: AppBannerType.info,
+          typeLabel: 'Note',
+          message: 'Confirm first',
+          actionLabel: 'Resend',
+          onAction: () => calls++,
+          actionEnabled: false,
+        ),
+      );
+
+      final action = find.widgetWithText(TextButton, 'Resend');
+      expect(action, findsOneWidget);
+      expect(tester.widget<TextButton>(action).onPressed, isNull);
+      expect(
+        tester
+            .getSemantics(action)
+            .getSemanticsData()
+            .flagsCollection
+            .isEnabled,
+        Tristate.isFalse,
+      );
+      await tester.tap(action, warnIfMissed: false);
+      expect(calls, 0);
+      handle.dispose();
+    });
+
     testWidgets('action calls back and is at least 48 dp', (tester) async {
       var calls = 0;
       await pumpWidgetUnderTest(