{{/* abono-billing-button — a POST form that opens Stripe's billing
     portal, where a subscriber changes their card or cancels. A POST
     because the handler mints a portal session.

     A reader may pay through more than one Stripe customer (see
     Service.Customers): render one button per customer, each posting its
     id as "customer", for Service.Portal.

     Keys:
       Action    string, required — where the form posts
       Customer  string, optional — the Stripe customer, posted as "customer"
       L         *ui.Localizer */}}
{{define "abono-billing-button"}}{{$L := localizer $}}<form rst-form-actions method="post" action="{{.Action}}">{{with opt . "Customer"}}<input type="hidden" name="customer" value="{{.}}">{{end}}<button type="submit" rst-btn>{{T $L "abono.button.manage"}}</button></form>{{end}}