fix(learn): a tab whose material is in its topics says so in the middle, as an empty tab does

The tab saying that its material is in the studyset's topics stood at the
top of the room below the tabs, while an empty tab's message stands in
its middle (SliverCentered), so switching between them moved the message
up and down (the developer's review, 9 Nov 2026). It stands in the middle
too now.

Whats-New: A studyset's tab whose material is in its topics now says so in the middle, as empty tabs do.
Whats-New-De: Ein Tab eines Lernsets, dessen Material in seinen Themen liegt, sagt das jetzt in der Mitte, wie leere Tabs.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

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

@@ -203,6 +203,15 @@ void main() {
 
       expect(find.text('The flashcards are in its topics'), findsOneWidget);
       expect(find.text('No flashcards yet'), findsNothing);
+      // In the middle of the room below the tabs, as an empty tab says it
+      // (the developer's review, 9 Nov 2026).
+      expect(
+        find.ancestor(
+          of: find.text('The flashcards are in its topics'),
+          matching: find.byType(SliverCentered),
+        ),
+        findsOneWidget,
+      );
       final kana = find.ancestor(
         of: find.text('Kana').last,
         matching: find.byType(ListTile),