Multi-base-currency household members

Support households where members use different base currencies. Each user sees their stats (cash flow, net worth, spending breakdowns) in their own base currency. Shared budgets normalize to the owner’s currency.

Today, sharing effectively requires matching base currencies — this lifts that restriction so cross-border couples and families can collaborate without forcing one side to switch their accounting currency.

Part of: https://moneymatter.featurebase.app/p/household-sharing-umbrella