Implement transaction simulation

Introduce mock transaction system to replace blockchain logic: - Add TypeScript models for Transaction, Currency, and TransactionStatus - Create mock service at src/services/transactionService.ts with send/getTransactions/getExchangeRate, etc. - Add SendMoney flow page for input, rate/fee display, and simulated processing - Update Home to show transaction history with simulated data - Update Pay page to remove wallet wiring and include Send Money / Request Money actions - Add helper components/types and routing support for new flow X-Lovable-Edit-ID: edt-834be6a1-37c6-4442-9ff1-cbf39f36409c