feat: display arbitrator address on escrow invoice detail

[MarcusDavidG]

Summary

Escrow invoices with an arbitrator set show no indication of the arbitrator address on the invoice detail page. Payers need to know who can resolve disputes.

Tasks

• On /invoice/[id], if invoice.arbitrator is set, display it in the escrow info section
• Truncated address with copy button and Stellar Explorer link

Acceptance criteria

• Arbitrator address visible on escrow invoice detail
• Copy-to-clipboard works