feat: show payout preview before payment confirmation

[MarcusDavidG]

Summary

Before a payer signs a payment transaction, show them the exact per-recipient breakdown using previewPayout(invoiceId, amount).

Design

• On the /pay/[id] page, after the payer enters an amount, call previewPayout and show a breakdown table: Recipient | Amount | Token
• Display while the payer reviews before clicking confirm
• Uses the same dust-correct rounding logic as the contract

Acceptance criteria

• Breakdown visible before signing on /pay/[id]
• Updates live as amount input changes (debounced)
• Handles split rules (Fixed, Percentage, Tiered) correctly