fix: add mounted guard before setState in _CopyButton._copyToClipboard

setState() was called after 'await Clipboard.setData()' without a
mounted check — throws if the widget is disposed during the clipboard
write (e.g. user navigates away from the code block screen).

Author: vietnguyentuan2019

lib/src/widgets/code_block_widget.dart

@@ -261,6 +261,7 @@ class _CopyButtonState extends State<_CopyButton> {
 
   Future<void> _copyToClipboard() async {
     await Clipboard.setData(ClipboardData(text: widget.code));
+    if (!mounted) return;
     setState(() => _copied = true);
 
     await Future.delayed(const Duration(seconds: 2));