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/l10n/halkyon_app_localizations_en.dart

@@ -193,6 +193,25 @@ class HalkyonAppLocalizationsEn extends HalkyonAppLocalizations {
   String get startNoGitHubAccount =>
       'No account here signs in with this GitHub account yet.';
 
+  @override
+  String get gitHubCreateAccountInstead =>
+      'Create an account with your email address and a password instead.';
+
+  @override
+  String get gitHubNoAccountSetUpPassword =>
+      'No account here signs in with this GitHub account yet. To keep your progress, set up a password instead.';
+
+  @override
+  String get gitHubUsePassword =>
+      'This account signs in with its email address and password now, not with GitHub.';
+
+  @override
+  String get gitHubDeprecated =>
+      'GitHub can\'t be added as a way to sign in any more: your email address and password are.';
+
+  @override
+  String get gitHubEnded => 'Sign-in with GitHub has ended on this instance.';
+
   @override
   String get startOtherInstance => 'Use another instance';