centered the episode text

lib/pages/reader/scaffold.dart

@@ -260,8 +260,10 @@ class _ReaderScaffoldState extends State<_ReaderScaffold> {
                   color: Theme.of(context).colorScheme.tertiaryContainer,
                   borderRadius: BorderRadius.circular(8),
                 ),
+                child: Center(
                   child: Text(text),
                 ),
+              ),
               const Spacer(),
               if (App.isWindows)
                 Tooltip(