Invoicing: Group-Specific Project Payment Progress (invoice by group)

When creating an invoice, include a dropdown selector for the estimate→group that the invoice pertains to.

And, allow the invoices page to be sorted by group — displaying each group’s estimate total, the outstanding “sent” amount, the amount paid so far, and the remaining estimate amount, with a list of the invoices pertaining to the group beneath each group’s heading. (similar to the “project value” display but per-group).