HOUSEBOT-079 Settlement Currency And NBG FX

Goal

Make household balances settle in a configurable household currency while preserving original entry currencies.

add a household settlementCurrency setting, defaulting to GEL
open new billing cycles in the settlement currency by default
keep rent rules in their original source currency
convert rent, utility bills, and purchase entries into the billing cycle currency for settlement
lock cycle exchange rates in the database once the configured reminder day has passed
use NBG as the FX source for GEL conversions
show both original and converted amounts in the mini app when they differ
default utility entry currency and bare purchase parsing currency to the household settlement currency

this slice does not add payment confirmation tracking yet
current FX behavior uses the configured reminder day as the target lock date
before the reminder day passes, the app may preview the latest available NBG rate without persisting it
after the reminder day passes, the rate is persisted per cycle and currency pair for deterministic future statements