docs: wire BeautifulUI playbook into aiui.template.yaml navigation

The shipped docs site is built by `aiui build` from aiui.template.yaml
(see ci.yml / deploy-pages.yml), not mkdocs.yml. Add a Frontend nav group
so the playbook is discoverable in the built site navigation.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

aiui.template.yaml

@@ -59,6 +59,11 @@ navigation:
             - typescript/runtime
             - typescript/nextjs
             - typescript/components
+        - group: "Frontend"
+          icon: "layout"
+          prefix: "frontend"
+          pages:
+            - frontend/beautifului
         - group: "Examples"
           icon: "folder-open"
           prefix: "examples"