feat(format): add locale-aware currency formatter and apply to receipt

[umohjosephemmanuel-cyber]

CLOSES #467

Description

Amounts were formatted ad hoc across the app — raw numbers in
TransactionSuccessReceipt.tsx, inline arithmetic in app/send/page.tsx, and
split percentages in CurrentMoneySplitWidget.tsx. This PR introduces a single
formatCurrency/formatAmount utility and applies it consistently across all
high-visibility financial surfaces.

Inconsistent number formatting (decimals, thousands separators, currency
symbols) on a financial product erodes trust and breaks for non-US locales.

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Changes

• lib/utils/format-currency.ts — new formatCurrency(amount, currency, locale,
  options) utility using Intl.NumberFormat, with graceful fallback (1,234.50
  USDC) for stablecoin codes that Intl does not recognize
• lib/utils/format-currency.test.ts — Vitest unit tests covering: USD/en-US,
  negative values, zero, USDC stablecoin fallback, es-ES locale separators, and
  formatAmount alias
• components/TransactionSuccessReceipt.tsx — applied formatCurrency to all
  displayed amounts
• components/CurrentMoneySplitWidget.tsx — applied formatter to split
  percentages/amounts
• app/send/page.tsx, AutomaticSplitCard.tsx, ReviewStep.tsx — applied
  formatter to send flow breakdown