Add support for Belarusian Ruble (BYN)

[lapytko]

The currency list doesn't include the Belarusian Ruble (BYN), so users in Belarus have to fall back on RUB or EUR, which throws off budgeting and exchange-rate math.

Currency: Belarusian Ruble — code BYN, 2 decimal places for amounts (standard, no special formatting needed).

Symbol note: the National Bank of Belarus approved a new official graphic symbol for the ruble in January 2026 (a Cyrillic "Б" with a horizontal bar). It's not yet in a released Unicode version, so it can't be rendered reliably today — BYN or Br/руб. should be used as the display symbol for now, with the new sign added later once font/Unicode support lands.

Important: the official exchange rate for BYN is quoted with 4 decimal digits, not 2. Rounding it to 2 decimals introduces real error over larger amounts, so the rate itself needs 4-decimal precision even though the currency amount stays at 2.

Adding BYN to the currency list (and ideally a be-BY regional preset) would let Belarusian households track their budget in their actual currency, with accurate conversions.

Link to official symbol: https://www.nbrb.by/coinsbanknotes/byn-ico

[ulsklyc]

Shipped in v2.11.0 🎉

BYN is selectable wherever a currency is: household preferences, subscriptions and shared expenses. Belarus also joins the region presets, so picking it fills in BYN, DD.MM.YYYY and a 24-hour clock in one step.

Thanks for the request, and for naming the code rather than just the currency.

[lapytko]

Thank you for implementing this feature. The app is really cool and useful.