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/simplelogin/widget/simplelogin_section_test.dart

@@ -1,6 +1,7 @@
 import 'package:flutter_test/flutter_test.dart';
 import 'package:halkyon_app/halkyon_app.dart';
 import 'package:halkyon_app/testing.dart';
+import 'package:halkyon_ui/halkyon_ui.dart';
 import 'package:material_ui/material_ui.dart';
 
 const password = 'correct horse battery';
@@ -46,10 +47,11 @@ void main() {
 
     expect(find.text('SimpleLogin'), findsOneWidget);
     expect(find.widgetWithText(ListTile, 'Add SimpleLogin'), findsOneWidget);
-    expect(
-      find.text('Sign in with SimpleLogin, or with your Proton account.'),
-      findsOneWidget,
-    );
+    const hint = 'Sign in with SimpleLogin, or with your Proton account.';
+    expect(find.text(hint), findsNothing, reason: 'behind its question mark');
+    await tester.tap(find.byType(HelpButton));
+    await tester.pumpAndSettle();
+    expect(find.text(hint), findsOneWidget);
   });
 
   testWidgets('nothing where the instance offers no SimpleLogin', (