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/bloc/session_bloc_test.dart

@@ -154,6 +154,37 @@ void main() {
     expect(gateway.gitHubSwitches, 0);
   });
 
+  test('an account that signs in with its password now, and GitHub having '
+      'ended, are each said so, signing nobody in', () async {
+    start(signedOut: true);
+    bloc.add(const SessionEventStarted());
+    await expectLater(bloc.stream, emits(const SessionStateSignedOut()));
+
+    gateway.gitHubError = const GitHubSignInFailed(
+      GitHubSignInProblem.usePassword,
+    );
+    bloc.add(const SessionEventGitHubRequested());
+    await expectLater(
+      bloc.stream,
+      emitsInOrder([
+        const SessionStateSignedOut(busy: true),
+        const SessionStateSignedOut(problem: SignedOutProblem.usePassword),
+      ]),
+    );
+
+    gateway.gitHubError = const GitHubSignInFailed(GitHubSignInProblem.ended);
+    bloc.add(const SessionEventGitHubRequested());
+    await expectLater(
+      bloc.stream,
+      emitsInOrder([
+        const SessionStateSignedOut(busy: true),
+        const SessionStateSignedOut(problem: SignedOutProblem.gitHubEnded),
+      ]),
+    );
+    expect(gateway.signIns, 0);
+    expect(gateway.gitHubSwitches, 0);
+  });
+
   test('a session the server ended asks how to start too', () async {
     const stored = SignedIn(authUserId: 'github-1', isIdentified: true);
     start(status: stored);