feat: add pagination support to getInvoicesByCreator and getInvoicesByPayer

[MarcusDavidG]

Summary

Both index methods return the full array. For power users with many invoices this could be large.

Proposal

Add optional slice params:

async getInvoicesByCreator(creator: string, opts?: { offset?: number; limit?: number }): Promise<number[]>

Slice the returned array client-side (contract returns full list — slicing is in the SDK).

Acceptance criteria

• Both methods accept optional offset and limit
• Backwards compatible — defaults return full array
• JSDoc updated