fix(app): GitHub's refusals say why, and no hint adds GitHub where passwords are offered

Where an instance offers password sign-in, GitHub is added to nobody (ADR
0017, D2), yet the start screen still said "then add GitHub" after a GitHub
account nobody has, and every refusal but an unknown account read
"Couldn't sign in with GitHub." (the end-to-end test of 21 Oct).

- The gateway tells from one read of the basics whether GitHub signs in and
  whether it is added to people (`gitHubOffer`, in place of
  `gitHubClientId`); the GitHub bloc keeps it (`Ready.linking`) for the
  profile, with why an attempt failed (`Ready.problem`).
- The instance's `usePassword`, `deprecated` and `ended` become problems of
  their own (`gitHubSignInProblemOf`), worded by `gitHubProblemText` and on
  the start screen; an anonymous person refused a link is told the GitHub
  account is nobody's and offered the password instead.
- Where passwords are offered, the start screen offers an account to create
  in place of the app's hint on adding GitHub.

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

@@ -28,6 +28,7 @@ void main() {
     WidgetTester tester, {
     bool usedBefore = false,
     bool wasSignedOut = false,
+    bool passwords = true,
   }) async {
     tester.view.physicalSize = const Size(412, 900);
     tester.view.devicePixelRatio = 1;
@@ -50,7 +51,7 @@ void main() {
           auth: AuthRepository(gateway, addresses),
           signedOut: signedOut,
           passwords: PasswordRepository(
-            FakePasswordGateway(gateway),
+            FakePasswordGateway(gateway)..offered = passwords,
             FakeAccountCrypto(),
             MemoryAccountKeyStore(),
             gateway,
@@ -113,7 +114,8 @@ void main() {
   });
 
   testWidgets('signed out, it asks how to start; a GitHub account nobody here '
-      "signs in with is said so, with Todo's ways in", (tester) async {
+      'signs in with is said so, with an account to create in its place '
+      'where passwords are offered (ADR 0017)', (tester) async {
     await start(tester, usedBefore: true, wasSignedOut: true);
 
     expect(find.text('Sign in with GitHub'), findsOneWidget);
@@ -123,6 +125,26 @@ void main() {
     await tester.tap(find.text('Sign in with GitHub'));
     await tester.pumpAndSettle();
 
+    expect(
+      find.text(
+        'No account here signs in with this GitHub account yet. Create an '
+        'account with your email address and a password instead.',
+      ),
+      findsOneWidget,
+    );
+  });
+
+  testWidgets("where no passwords are offered, with Todo's ways in", (
+    tester,
+  ) async {
+    await start(tester, usedBefore: true, wasSignedOut: true, passwords: false);
+
+    gateway.gitHubError = const GitHubSignInFailed(
+      GitHubSignInProblem.unknownAccount,
+    );
+    await tester.tap(find.text('Sign in with GitHub'));
+    await tester.pumpAndSettle();
+
     expect(
       find.text(
         'No account here signs in with this GitHub account yet. Start '