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/lib/src/feature/studyset/widget/studyset_rail.dart

@@ -23,7 +23,9 @@ import 'package:material_ui/material_ui.dart';
 /// topics are rearranged. Below, the topics in their order, each opening its
 /// own screen on top of this one, with the study it invites and its options;
 /// a switch rearranges them by dragging. New sub-studyset stays at the foot,
-/// across from [footEnd]. Without topics, an invitation to create the first.
+/// across from [footEnd], with or without topics, so the foot looks the same
+/// in every studyset (decided 2026-10-08). Without topics, a word on what
+/// they are for in their place.
 class const StudysetRail({
   /// The studyset whose topics these are.
   required final Studyset parent,
@@ -113,10 +115,7 @@ class _StudysetRailState() extends State<StudysetRail> {
           _ => const <String, StudyProgress>{},
         };
         final addMaterial = widget.onAddMaterial;
-        final newSubStudyset =
-            state is StudysetListStateLoaded &&
-            topics.isNotEmpty &&
-            !_reordering;
+        final newSubStudyset = state is StudysetListStateLoaded && !_reordering;
         final footEnd = widget.footEnd;
         return Column(
           crossAxisAlignment: CrossAxisAlignment.stretch,
@@ -174,9 +173,7 @@ class _StudysetRailState() extends State<StudysetRail> {
                 StudysetListStateFailed() => Center(
                   child: Text(l10n.libraryUnreadable),
                 ),
-                StudysetListStateLoaded(studysets: []) => _NoTopics(
-                  onCreate: _create,
-                ),
+                StudysetListStateLoaded(studysets: []) => const _NoTopics(),
                 StudysetListStateLoaded() when _reordering => Scrollbar(
                   controller: _scroll,
                   thumbVisibility: true,
@@ -248,8 +245,8 @@ class _StudysetRailState() extends State<StudysetRail> {
               },
             ),
             // At the foot, below a line, however long the list: New
-            // sub-studyset while topics are listed (without, the invitation
-            // in their place has it), and what the rail is given to end on.
+            // sub-studyset once the topics are read, and what the rail is
+            // given to end on.
             if (newSubStudyset || footEnd != null)
               DecoratedBox(
                 decoration: BoxDecoration(
@@ -582,9 +579,9 @@ class const StudyActionIcon({
   }
 }
 
-/// No topics yet: an invitation to create the first.
-class const _NoTopics({required final VoidCallback onCreate})
-    extends StatelessWidget {
+/// No topics yet, and what they are for; New sub-studyset at the foot
+/// creates the first.
+class const _NoTopics() extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final l10n = AppLocalizations.of(context);
@@ -606,12 +603,6 @@ class const _NoTopics({required final VoidCallback onCreate})
               color: theme.colorScheme.onSurfaceVariant,
             ),
           ),
-          const SizedBox(height: 12),
-          FilledButton.tonalIcon(
-            onPressed: onCreate,
-            icon: const Icon(LucideIcons.plus),
-            label: Text(l10n.studysetNewSubStudyset),
-          ),
         ],
       ),
     );

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 {