refactor(empty-state): 空状态支持内容居中

test/watch/watch_records_page_test.dart

@@ -209,6 +209,27 @@ void main() {
     expect(find.text('The Signal'), findsOneWidget);
   });
 
+  testWidgets('should_center_empty_state_in_remaining_page_space', (
+    tester,
+  ) async {
+    final auth = await _auth(signedIn: true);
+    await _pumpPage(
+      tester,
+      auth: auth,
+      history: CountingWatchHistory(const []),
+      catalog: FakeCatalogRepository(),
+    );
+
+    final viewport = tester.getRect(
+      find.byKey(const Key('watch-records-empty-scroll')),
+    );
+    final content = tester.getRect(
+      find.byKey(const Key('empty-state-content')),
+    );
+    expect(content.center.dx, closeTo(viewport.center.dx, 0.1));
+    expect(content.center.dy, closeTo(viewport.center.dy, 0.1));
+  });
+
   testWidgets('should_remove_and_clear_device_history_when_signed_out', (
     tester,
   ) async {