Split calculator

One shared bill or set of costs: a title, the currency (ISO 4217, e.g. EUR) and optionally the locale numbers are written in, the people (a short id and a name each), the items (a label and an amount in major units, e.g. 24.5 for €24.50) and how each is shared — equally among everyone or among the listed people, by weighted shares, or by exact amounts per person. Optional: charges and discounts (a percent of the item subtotal or a fixed amount, negative for a discount; `included: true` for a tax already in the prices, shown but not added), a tip with presets the visitor can pick from, the payments people made (one may omit its amount to mean 'paid the rest'), rounding for money that changes hands, and a short note. All arithmetic is done in whole minor units (cents), with leftover cents given out fairly, never lost.