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/lib/src/auth/bloc/session_bloc.dart

@@ -102,9 +102,14 @@ class SessionBloc(final AuthRepository _auth, final SignedOutMark _mark)
       _offerPassword = false;
       emit(
         SessionStateSignedOut(
-          problem: e.problem == GitHubSignInProblem.unknownAccount
-              ? SignedOutProblem.noGitHubAccount
-              : SignedOutProblem.failed,
+          problem: switch (e.problem) {
+            GitHubSignInProblem.unknownAccount =>
+              SignedOutProblem.noGitHubAccount,
+            GitHubSignInProblem.usePassword => SignedOutProblem.usePassword,
+            GitHubSignInProblem.ended => SignedOutProblem.gitHubEnded,
+            GitHubSignInProblem.deprecated ||
+            GitHubSignInProblem.refused => SignedOutProblem.failed,
+          },
         ),
       );
     } on Object catch (error, st) {
@@ -246,9 +251,16 @@ final class const SessionStateSignedOut({
 /// Why a way to start failed.
 enum SignedOutProblem() {
   /// No account has the GitHub account yet: start without one, then add
-  /// GitHub.
+  /// GitHub, or, where passwords are offered, create an account.
   noGitHubAccount,
 
+  /// The GitHub account's person signs in with their address and password
+  /// now (ADR 0017).
+  usePassword,
+
+  /// GitHub signs nobody in here any more.
+  gitHubEnded,
+
   /// Anything else, as no connection.
   failed,
 }