Three related changes on /member/account:
1. Payment History section now renders when contributionAmount > 0 OR
past payments exist. Previously a paid member who switched to $0 lost
visibility of their own past charges.
2. New "Next charge: $X on DATE" row renders above the transaction list
when nextPaymentDate is available, using --candle dashed border.
3. server/api/helcim/subscription.get.js now reads dateBilling from
Helcim's GET response and handles data as either object or array.
Helcim's real shape is {data: {id, dateBilling, ...}} — the old code
expected {data: [{nextBillingDate}]} and returned empty strings, so
the Membership-card "Next payment" row never rendered for members
whose cached date was missing. subscription.post.js and
update-contribution.post.js have the same wrong field name in their
CREATE flows; left for a follow-up — the GET refresh masks it.
Manual edit-flow and admin-flow tests also recorded in
docs/LAUNCH_READINESS.md.
Persist nextBillingDate on subscription create/update; unset on
cancel or downgrade to free. Account page displays the cached
date and lazily refreshes from Helcim when the cached value is
within 24h of now (or missing).
