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>

app/(app)/products/[id]/layout.tsx

@@ -0,0 +1,25 @@
+import { redirect, notFound } from 'next/navigation'
+import { getSession } from '@/lib/auth'
+import { getAccessibleProduct } from '@/lib/product-access'
+import { SetCurrentProduct } from '@/components/shared/set-current-product'
+
+interface Props {
+  children: React.ReactNode
+  params: Promise<{ id: string }>
+}
+
+export default async function ProductLayout({ children, params }: Props) {
+  const { id } = await params
+  const session = await getSession()
+  if (!session.userId) redirect('/login')
+
+  const product = await getAccessibleProduct(id, session.userId)
+  if (!product) notFound()
+
+  return (
+    <>
+      <SetCurrentProduct id={id} name={product.name} />
+      {children}
+    </>
+  )
+}