feat(learn,app): explanations wait behind a question mark beside their title

What explains a setting or a section, rather than what is the person's
own, moves behind halkyon_ui's HelpButton, beside its title through the
new ExplainedTitle (the developer's review of 3 Nov 2026): in Learn's
settings (citations, the Pomodoro timer, grammar, the web search,
Jellyfin, the device's dictionaries, counting one's learning), the study
settings, export's progress, the Jellyfin tags and the study button's
back first; in halkyon_app, setting up a password, adding SimpleLogin,
own keys not allowed, the profile page's link back and the operator's
grant sheet.

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/assistant/widget/assistants_card_test.dart

@@ -406,14 +406,16 @@ void main() {
       ),
     );
     await show(tester);
-
-    expect(
-      find.text(
+    const why =
         "Whoever runs test.example.org doesn't allow own keys here, so the "
-        "app doesn't ask these.",
-      ),
-      findsOneWidget,
-    );
+        "app doesn't ask these.";
+
+    expect(find.text(why), findsNothing, reason: 'behind its question mark');
+    await tester.tap(find.byType(HelpButton));
+    await tester.pumpAndSettle();
+    expect(find.text(why), findsOneWidget);
+    await tester.tap(find.text('OK'));
+    await tester.pumpAndSettle();
     expect(find.text('Add an assistant with your own key'), findsNothing);
   });