Helcim returns next-charge as `dateBilling` on POST /subscriptions, but the
two CREATE sites were reading `subscription.nextBillingDate`, leaving
`member.nextBillingDate` empty after every signup and free→paid upgrade.
The lazy refresh in subscription.get.js (which already accepts both shapes)
masked it on next account-page load, so renders eventually populated — but
the success response we returned to the client also had `nextBillingDate:
undefined`. Mirror the GET-side resolution at both CREATE sites: prefer
`dateBilling`, fall back to `nextBillingDate`. Existing Number.isNaN guard
unchanged; defensively rejects malformed strings from either field.
Jennie Robinson Faber
cf59931814