feat(ui): the question mark's explanation offers a link beside OK

HelpButton takes an action, a label and what it does, shown beside OK where
there is more to read elsewhere, as what a map's source keeps (Train's ADR
0030, D63).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_ui/lib/src/widget/help_button.dart

@@ -6,11 +6,13 @@ import 'package:material_ui/material_ui.dart';
 /// A question mark beside a section's or a setting's title that keeps its
 /// explanation out of the everyday view: a tap opens [message] in a small
 /// dialog, under [title] where given (decided 3 Nov 2026: what explains,
-/// rather than what is the person's own, is one tap away). For a field's
+/// rather than what is the person's own, is one tap away), with [action]
-/// short hint, [FieldInfoButton]'s tooltip.
+/// beside OK where there is more to read elsewhere. For a field's short
+/// hint, [FieldInfoButton]'s tooltip.
 class const HelpButton(
   final String message, {
   final String? title,
+  final ({String label, VoidCallback onPressed})? action,
   super.key,
 }) extends StatelessWidget {
   @override
@@ -26,6 +28,8 @@ class const HelpButton(
         title: title == null ? null : Text(title!),
         content: SingleChildScrollView(child: Text(message)),
         actions: [
+          if (action case (:final label, :final onPressed))
+            TextButton(onPressed: onPressed, child: Text(label)),
           TextButton(
             onPressed: () => Navigator.of(context).pop(),
             child: Text(MaterialLocalizations.of(context).okButtonLabel),

packages/halkyon_ui/test/widget/help_button_test.dart

@@ -26,4 +26,22 @@ void main() {
     await tester.pumpAndSettle();
     expect(find.text('How sources are cited.'), findsNothing);
   });
+  testWidgets('an action offered stands beside OK', (tester) async {
+    var read = false;
+    await tester.pumpWidget(
+      testApp(
+        Scaffold(
+          body: HelpButton(
+            'Map by OpenFreeMap.',
+            action: (label: 'What it keeps', onPressed: () => read = true),
+          ),
+        ),
+      ),
+    );
+    await tester.tap(find.byType(HelpButton));
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('What it keeps'));
+    expect(read, isTrue);
+    expect(find.text('OK'), findsOneWidget);
+  });
 }