feat(locale): persist household and member preferences

packages/domain/src/index.ts

@@ -2,6 +2,7 @@ export { BillingPeriod } from './billing-period'
 export { DOMAIN_ERROR_CODE, DomainError } from './errors'
 export { BillingCycleId, HouseholdId, MemberId, PurchaseEntryId } from './ids'
 export { CURRENCIES, Money } from './money'
+export { normalizeSupportedLocale, SUPPORTED_LOCALES } from './locale'
 export {
   Temporal,
   instantFromDatabaseValue,
@@ -13,6 +14,7 @@ export {
   nowInstant
 } from './time'
 export type { CurrencyCode } from './money'
+export type { SupportedLocale } from './locale'
 export type { Instant } from './time'
 export type {
   SettlementInput,

packages/domain/src/locale.ts

@@ -0,0 +1,14 @@
+export const SUPPORTED_LOCALES = ['en', 'ru'] as const
+
+export type SupportedLocale = (typeof SUPPORTED_LOCALES)[number]
+
+export function normalizeSupportedLocale(value?: string | null): SupportedLocale | null {
+  const normalized = value?.trim().toLowerCase()
+  if (!normalized) {
+    return null
+  }
+
+  return (SUPPORTED_LOCALES as readonly string[]).includes(normalized)
+    ? (normalized as SupportedLocale)
+    : null
+}