Pay by card from the membership invoice itself

Members pay dues where the invoice is, so the invoice now carries the link. Nothing to set up: once your Stripe account is connected, every membership invoice with a balance gets it.

• The invoice PDF prints a clickable pay-by-card line under the balance, the same way event invoices already did. Forward it or print it and the link still works.
• The invoice email gets a Pay invoice online button. Receipts for paid invoices have no button.
• The Renew now button on the five renewal reminders opens card payment for that renewal, with no member sign-in. Before, it led to the member hub sign-in page, which most members have never used.