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/test/auth/widget/session_gate_test.dart

@@ -113,8 +113,9 @@ void main() {
     await tester.pumpAndSettle();
     expect(
       find.text(
+        // The package's sentence, then the app's on where GitHub is added.
         'No account here signs in with this GitHub account yet. Start '
-        'without an account, then add GitHub in Profile.',
+        'without an account, then add GitHub in Settings.',
       ),
       findsOneWidget,
     );