feat: add TypeScript types for DisputeState and InvoiceStats

[MarcusDavidG]

Summary

getEscrowState and getInvoiceStats return structured objects but there are no exported TypeScript interfaces for them.

Types to add and export from index.ts

export interface DisputeState { releaseAt: number; disputed: boolean; disputedAt: number }
export interface InvoiceStats { funded: bigint; total: bigint; paymentCount: number; uniquePayers: number; completionBps: number }

Acceptance criteria

• Both interfaces defined in client.ts and exported from index.ts
• getInvoiceStats and getEscrowState return these types