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_localizations.dart

@@ -2830,6 +2830,12 @@ abstract class AppLocalizations {
   /// **'Progress is saved on this device. Sign in to keep it.'**
   String get profileAnonymous;
 
+  /// No description provided for @profileAnonymousSimpleLogin.
+  ///
+  /// In en, this message translates to:
+  /// **'Progress is saved on this device. Add SimpleLogin to keep it.'**
+  String get profileAnonymousSimpleLogin;
+
   /// No description provided for @profileSignInWithGitHub.
   ///
   /// In en, this message translates to: