feat: show active product name in navbar, links to product page

Sub-layout sets product in Zustand store; NavBar reads it.
getAccessibleProduct wrapped with React cache to avoid double DB call.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/shared/set-current-product.tsx

@@ -0,0 +1,15 @@
+'use client'
+
+import { useEffect } from 'react'
+import { useProductStore } from '@/stores/product-store'
+
+export function SetCurrentProduct({ id, name }: { id: string; name: string }) {
+  const { setCurrentProduct, clearCurrentProduct } = useProductStore()
+
+  useEffect(() => {
+    setCurrentProduct(id, name)
+    return () => clearCurrentProduct()
+  }, [id, name, setCurrentProduct, clearCurrentProduct])
+
+  return null
+}