Fix copy button width on mobile by scoping 100% width to button-group only (#179)

Co-authored-by: Claude <noreply@anthropic.com>

Author: simonw

quickjs.html

@@ -187,7 +187,7 @@
             .button-group {
                 flex-direction: column;
             }
-            button {
+            .button-group button {
                 width: 100%;
             }
         }