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>

packages/halkyon_app/test/auth/widget/session_gate_test.dart

@@ -98,7 +98,8 @@ void main() {
   });
 
   testWidgets('from there, GitHub signs in its person; a GitHub account '
-      'nobody here has yet is said so', (tester) async {
+      'nobody here has yet is said so, with an account to create in its '
+      'place where passwords are offered (ADR 0017)', (tester) async {
     await dependencies.signedOut.write(signedOut: true);
     await start(tester);
 
@@ -109,12 +110,16 @@ void main() {
     await tester.pumpAndSettle();
     expect(
       find.text(
-        // The package's sentence, then the app's on where GitHub is added.
-        'No account here signs in with this GitHub account yet. Start '
-        'without an account, then add GitHub in Settings.',
+        'No account here signs in with this GitHub account yet. Create an '
+        'account with your email address and a password instead.',
       ),
       findsOneWidget,
     );
+    expect(
+      find.textContaining('add GitHub'),
+      findsNothing,
+      reason: 'GitHub is added to nobody where passwords are offered',
+    );
 
     gateway.gitHubError = null;
     await tester.tap(find.text('Sign in with GitHub'));
@@ -140,6 +145,55 @@ void main() {
     expect(find.text('App of github-person'), findsOneWidget);
   });
 
+  testWidgets('where no passwords are offered, a GitHub account nobody here '
+      'has yet is said so with where the app adds GitHub', (tester) async {
+    dependencies.passwordGateway.offered = false;
+    await dependencies.signedOut.write(signedOut: true);
+    await start(tester);
+
+    gateway.gitHubError = const GitHubSignInFailed(
+      GitHubSignInProblem.unknownAccount,
+    );
+    await tester.tap(find.text('Sign in with GitHub'));
+    await tester.pumpAndSettle();
+    expect(
+      find.text(
+        // The package's sentence, then the app's on where GitHub is added.
+        'No account here signs in with this GitHub account yet. Start '
+        'without an account, then add GitHub in Settings.',
+      ),
+      findsOneWidget,
+    );
+  });
+
+  testWidgets('an account that signs in with its password, and GitHub having '
+      'ended, are each said so', (tester) async {
+    await dependencies.signedOut.write(signedOut: true);
+    await start(tester);
+
+    gateway.gitHubError = const GitHubSignInFailed(
+      GitHubSignInProblem.usePassword,
+    );
+    await tester.tap(find.text('Sign in with GitHub'));
+    await tester.pumpAndSettle();
+    expect(
+      find.text(
+        'This account signs in with its email address and password now, '
+        'not with GitHub.',
+      ),
+      findsOneWidget,
+    );
+
+    gateway.gitHubError = const GitHubSignInFailed(GitHubSignInProblem.ended);
+    await tester.tap(find.text('Sign in with GitHub'));
+    await tester.pumpAndSettle();
+    expect(
+      find.text('Sign-in with GitHub has ended on this instance.'),
+      findsOneWidget,
+    );
+    expect(find.text("Couldn't sign in with GitHub."), findsNothing);
+  });
+
   testWidgets('offers only the ways the instance has', (tester) async {
     gateway.offersGitHub = false;
     dependencies.passwordGateway.offered = false;