[FE-55] Implement multi-currency price display toggle

[mftee]

Overview

Add a currency toggle that lets users switch between USD, EUR, GBP, and NGN for all price displays across the platform. All work goes in frontend/sandbox/.

Acceptance Criteria

• Component at frontend/sandbox/components/CurrencyToggle.tsx
• Dropdown selector in the header: USD, EUR, GBP, NGN
• Selected currency stored in Zustand store and persisted in localStorage
• All price components read from the store and convert using mock exchange rates
• useCurrency() hook exposes currency, convert(amount: number), format(amount: number)
• Demo page at frontend/sandbox/page.tsx shows prices in all four currencies