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>

stores/product-store.ts

@@ -0,0 +1,13 @@
+import { create } from 'zustand'
+
+interface ProductStore {
+  currentProduct: { id: string; name: string } | null
+  setCurrentProduct: (id: string, name: string) => void
+  clearCurrentProduct: () => void
+}
+
+export const useProductStore = create<ProductStore>((set) => ({
+  currentProduct: null,
+  setCurrentProduct: (id, name) => set({ currentProduct: { id, name } }),
+  clearCurrentProduct: () => set({ currentProduct: null }),
+}))