fix(showcase/shell-docs): drop @copilotkitnext/react CSS import

shell-docs renders pure MDX docs with iframed live demos (via <InlineDemo>
and <IframeSwitcher>) — it never instantiates the live copilot runtime
inline, so it doesn't need @copilotkitnext/react or its styles. Dropping
the import also removes the package from shell-docs's dep closure, keeping
the docs shell lean.

Author: jpr5

showcase/shell-docs/src/app/globals.css

@@ -1,5 +1,4 @@
 @import "tailwindcss";
-@import "@copilotkitnext/react/styles.css";
 @import "highlight.js/styles/github.css";
 
 :root {