feat(app): adding SimpleLogin to an account (ADR 0020, D21, D23)

halkyon_app reads where the device's person stands with SimpleLogin
(nothing to show, linked, or missing) and shows it in SimpleLoginSection,
which each app places above the password part: Add SimpleLogin, once
added one more way in. GitHub is added to nobody where SimpleLogin is
offered, and Learn's Profile offers an anonymous person SimpleLogin
alone there (D23).

No cut-off day, reminder or banner (D24): with the developer the only
person on learn.halkyon.eu, the cut-off is the operator taking GitHub's
OAuth app out of the deploy's settings once that account has SimpleLogin.
ADR 0020 records it and drops step 5.

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

apps/learn/halkyon_learn_flutter/test/feature/auth/widget/simplelogin_profile_test.dart

@@ -0,0 +1,52 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/testing.dart';
+
+import '../../../support/pump_app.dart';
+import '../../../support/test_dependencies.dart';
+
+// Where the instance offers SimpleLogin (ADR 0020, D21, D23): it alone keeps
+// an anonymous account.
+void main() {
+  late FakeAuthGateway auth;
+  late FakeSimpleLoginGateway simpleLogin;
+
+  setUp(() {
+    auth = FakeAuthGateway();
+    simpleLogin = FakeSimpleLoginGateway(auth);
+  });
+
+  Future<void> start(WidgetTester tester) => pumpApp(
+    tester,
+    testDependencies(authGateway: auth, simpleLoginGateway: simpleLogin),
+  );
+
+  testWidgets('an anonymous person is offered SimpleLogin alone, not GitHub '
+      '(D23)', (tester) async {
+    await start(tester);
+    await openTab(tester, 'Profile');
+
+    expect(
+      find.text(
+        'Progress is saved on this device. Add SimpleLogin to keep it.',
+      ),
+      findsOneWidget,
+    );
+    expect(find.text('Sign in with GitHub'), findsNothing);
+    expect(find.text('Add SimpleLogin'), findsWidgets);
+  });
+
+  testWidgets('without SimpleLogin, GitHub keeps the account as before', (
+    tester,
+  ) async {
+    simpleLogin.isOffered = false;
+    await start(tester);
+    await openTab(tester, 'Profile');
+
+    expect(
+      find.text('Progress is saved on this device. Sign in to keep it.'),
+      findsOneWidget,
+    );
+    expect(find.text('Sign in with GitHub'), findsOneWidget);
+    expect(find.textContaining('SimpleLogin'), findsNothing);
+  });
+}