feat: implement `useHorizonSSE` hook for real-time transaction streaming

[rayeberechi]

Summary

useTransactions currently does a one-shot Horizon fetch. Horizon supports Server-Sent Events (SSE) for streaming new transactions in real time. This hook should wrap that capability.

Implementation Steps

1. Create src/hooks/useHorizonSSE.ts.
2. Signature: useHorizonSSE(publicKey: string | null, network: StellarNetwork, onRecord: (tx: TransactionRecord) => void): { close: () => void }.
3. Use Horizon.Server.transactions().forAccount(publicKey).cursor('now').stream({ onmessage, onerror }) from @stellar/stellar-sdk.
4. Call close() in the useEffect cleanup to prevent memory leaks.
5. Map the raw Horizon record to TransactionRecord using the same mapping logic as useTransactions.
6. Export from src/index.ts.
7. Write a unit test using vi.mock('@stellar/stellar-sdk') to simulate the stream emitting one record and verify onRecord is called.

Definition of Done

• Hook opens SSE stream on mount and closes on unmount.
• onRecord called with correctly typed TransactionRecord.
• No memory leaks (cleanup verified in test).
• TypeScript strict mode passes.