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_en.arb

@@ -167,6 +167,17 @@
   "startAnonymous": "Progress is saved on this device. Sign in to keep it.",
   "startGitHubFailed": "Couldn't sign in with GitHub.",
   "startNoGitHubAccount": "No account here signs in with this GitHub account yet.",
+  "gitHubCreateAccountInstead": "Create an account with your email address and a password instead.",
+  "@gitHubCreateAccountInstead": {
+    "description": "Said after a GitHub account nobody on the instance signs in with, where people sign in with a password and GitHub is added to nobody (ADR 0017)."
+  },
+  "gitHubNoAccountSetUpPassword": "No account here signs in with this GitHub account yet. To keep your progress, set up a password instead.",
+  "@gitHubNoAccountSetUpPassword": {
+    "description": "An anonymous person signed in with a GitHub account nobody here has, where GitHub is added to nobody: the password set up below keeps their account."
+  },
+  "gitHubUsePassword": "This account signs in with its email address and password now, not with GitHub.",
+  "gitHubDeprecated": "GitHub can't be added as a way to sign in any more: your email address and password are.",
+  "gitHubEnded": "Sign-in with GitHub has ended on this instance.",
   "startOtherInstance": "Use another instance",
   "syncTitle": "Sync",
   "syncNow": "Sync now",