feat(app): the topics' foot the same with or without topics

Without sub-studysets, the side bar and the drawer showed New
sub-studyset in the invitation at the top and the bibliography alone at
the foot. Now New sub-studyset always sits at the foot's start, across
from the bibliography, once the topics are read (not while reordering);
the invitation keeps its words, without a button of its own (decided
2026-10-08).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/test/feature/studyset/widget/studyset_screen_test.dart

@@ -442,6 +442,28 @@ void main() {
     expect(bibliography.right, moreOrLessEquals(rail.right - 16));
   });
 
+  testWidgets('without topics, the foot is the same: one New sub-studyset '
+      'at its start, the bibliography at its end', (tester) async {
+    stored('japanese', 'Japanese');
+    await showApp(tester, location: RoutePaths.studyset('japanese'));
+    await tester.pumpAndSettle();
+    expect(find.text('No sub-studysets yet.'), findsOneWidget);
+
+    expect(find.text('New sub-studyset'), findsOneWidget);
+    final create = tester.getRect(
+      find.ancestor(
+        of: find.text('New sub-studyset'),
+        matching: find.byWidgetPredicate((widget) => widget is FilledButton),
+      ),
+    );
+    final bibliography = tester.getRect(find.byTooltip('Bibliography'));
+    expect(bibliography.center.dy, moreOrLessEquals(create.center.dy));
+    expect(
+      create.top,
+      greaterThan(tester.getRect(find.text('No sub-studysets yet.')).bottom),
+    );
+  });
+
   testWidgets('on a phone, the bibliography shares the foot of the topics '
       'with New sub-studyset: that at the start, the bibliography at the '
       'end', (tester) async {