fix(webapp): note promo credits apply to the Free plan

The /promo page advertised credits without stating they attach to the
free-plan usage entitlement, so a visitor who picked a paid plan would
lose them silently. Make the Free-plan scope explicit on the page.

Author: matt-aitken

apps/webapp/app/routes/promo.tsx

@@ -158,7 +158,7 @@ export default function PromoPage() {
             </Header2>
             {data.view === "valid" ? (
               <Paragraph variant="base" spacing>
-                These are only available for new accounts.
+                These are only available for new accounts on the Free plan.
                 {formatExpiry(data.expiresAt)
                   ? ` The credits expire on ${formatExpiry(data.expiresAt)}.`
                   : ""}