refactor(halkyon_app): the app says where GitHub is added later

The start screen's sentence after a GitHub account nobody here signs in
with named Learn's Profile screen. The package now says only that no
account signs in with it; the app's frame adds the rest as a sentence
of its own (thenAddGitHub), since the place takes its language's
grammar ("im Profil"). Learn's reads as before, which a test now pins.

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

@@ -146,7 +146,7 @@ class HalkyonAppLocalizationsEn extends HalkyonAppLocalizations {
 
   @override
   String get startNoGitHubAccount =>
-      'No account here signs in with this GitHub account yet. Start without an account, then add GitHub in Profile.';
+      'No account here signs in with this GitHub account yet.';
 
   @override
   String get startOtherInstance => 'Use another instance';