Billing information is accessible in account settings

A subscription product with no billing page in account settings signals to users that the product is unfinished or abandoned — they cannot see their current plan, next charge date, or payment method, so they have no way to verify they are being billed correctly. This drives trust-based churn and triggers chargebacks from users who forgot they were subscribed because the product never surfaced their billing state in context.

Add a billing page at app/settings/billing/page.tsx that renders the user's current plan, subscription status, and a form that POSTs to the Customer Portal route. Link it from the main settings navigation so users can reach it without knowing the URL:

export default async function BillingPage() {
  const user = await getCurrentUser()
  return (<form action="/api/billing/portal" method="POST"><button>Manage Billing</button></form>)
}