feat(app): adding SimpleLogin to an account (ADR 0020, D21, D23)

halkyon_app reads where the device's person stands with SimpleLogin
(nothing to show, linked, or missing) and shows it in SimpleLoginSection,
which each app places above the password part: Add SimpleLogin, once
added one more way in. GitHub is added to nobody where SimpleLogin is
offered, and Learn's Profile offers an anonymous person SimpleLogin
alone there (D23).

No cut-off day, reminder or banner (D24): with the developer the only
person on learn.halkyon.eu, the cut-off is the operator taking GitHub's
OAuth app out of the deploy's settings once that account has SimpleLogin.
ADR 0020 records it and drops step 5.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -470,6 +470,7 @@
   "profileSettingsTooltip": "Einstellungen",
   "profileIdentified": "Dein Fortschritt wird in deinem Konto gespeichert.",
   "profileAnonymous": "Dein Fortschritt wird auf diesem Gerät gespeichert. Melde dich an, um ihn zu behalten.",
+  "profileAnonymousSimpleLogin": "Dein Fortschritt wird auf diesem Gerät gespeichert. Füge SimpleLogin hinzu, um ihn zu behalten.",
   "profileSignInWithGitHub": "Mit GitHub anmelden",
   "profileSignInFailed": "Anmeldung mit GitHub fehlgeschlagen.",
   "profileContinueInBrowser": "Fahre in deinem Browser fort.",