fix(home): restore showcase carousel and fix animation clipping (#153)

* fix(home): restore showcase carousel and fix entrance animation clipping

Reverts the homepage from the live DemoChat (with "Try Tool UI" buttons)
back to the original ChatShowcase carousel. Removes the "Create your own
Tool UI" CTA that linked to the broken builder. Fixes the faux chat shell
entrance animation being clipped by the nav bar — caused by overflow-hidden
being re-added to the ContentLayout wrapper in 4546c247, undoing the
intentional fix from c992a96a.

Closes #151

* style: fix oxfmt formatting in component-preview-shell