Split expenses

A group's shared expenses, with no accounts: the link is the invitation, and whoever opens it is in. Each expense is one node — who paid, how much, split among whom — and nothing else is ever stored. Every balance and every "who pays whom" below is derived on the spot from the whole set, in integer cents, so the totals always sum to zero.

That is why this is the most honest sync demo in the catalogue: open the same link in two windows and they either show the same balances, or you can see they have not converged yet — a balance is a sum over everything that arrived. Settling up writes one more node of the same kind, and the books close by themselves.