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/todo/halkyon_todo_flutter/test/app/todo_app_test.dart

@@ -29,6 +29,7 @@ void main() {
     bool usedBefore = false,
     bool wasSignedOut = false,
     bool passwords = true,
+    FakeSimpleLoginGateway? simpleLogin,
   }) async {
     tester.view.physicalSize = const Size(412, 900);
     tester.view.devicePixelRatio = 1;
@@ -57,9 +58,10 @@ void main() {
             gateway,
             addresses,
           ),
-          // Offered by no instance here: halkyon_app's tests test its sheet.
+          // Offered by no instance here but where a test gives one:
+          // halkyon_app's tests test its sheet.
           simpleLogin: SimpleLoginRepository(
-            FakeSimpleLoginGateway(gateway)..isOffered = false,
+            simpleLogin ?? (FakeSimpleLoginGateway(gateway)..isOffered = false),
             FakeOidcBrowser(),
             FakeAccountCrypto(),
             MemoryAccountKeyStore(),
@@ -184,6 +186,18 @@ void main() {
     expect(find.text('Sign in with GitHub'), findsOneWidget);
   });
 
+  testWidgets('where the instance offers SimpleLogin, the account sheet '
+      'adds it (ADR 0020, D21)', (tester) async {
+    await start(
+      tester,
+      usedBefore: true,
+      simpleLogin: FakeSimpleLoginGateway(gateway),
+    );
+
+    await openAccount(tester);
+    expect(find.widgetWithText(ListTile, 'Add SimpleLogin'), findsOneWidget);
+  });
+
   testWidgets('deleting the account asks first; Cancel keeps it, Delete '
       'deletes it and asks how to start', (tester) async {
     await start(tester, usedBefore: true);