feat: add react hooks package @stellar-sharpy/react

[MarcusDavidG]

Summary

A React hooks wrapper would let app developers integrate Sharpy with minimal boilerplate.

Proposed hooks

useInvoice(id) // fetches invoice, auto-refreshes
useCreateInvoice() // returns { create, loading, error }
useWallet() // publicKey, connect, disconnect
useInvoicesByCreator(creator) // paginated invoice list

Acceptance criteria

• New package packages/react in the monorepo (or separate repo)
• Hooks use SharpyClient internally
• Published as @stellar-sharpy/react