Settle Split

Mark a bill split as squared up between the two of you.

Settling is a bookkeeping fact, not a movement of money. earmarkIQ cannot make a transfer between two people's banks and cannot reliably observe one, so the only thing it can honestly record is that the two of you consider this settled — which removes the row from the outstanding balance and nothing else.

It deliberately does NOT touch budget spend: _calculate_partner_spend sums what each person actually spent, and a transfer between partners is not spending by either of them. Matching a settlement to a real bank transfer would need partner-to-partner transfer detection and is a separate piece of work.