fix(pay-request): lock amount/destination in SendForm and re-validate on submit (#26)

[ghzhost]

Closes #26

Problem

PayRequest.tsx previously passed requested amount and destination to SendForm via defaultValues, leaving the inputs editable by the payer before quote/payment submission. Additionally, if the payment request status changed (e.g., cancelled or expired) or if fields were modified, the submission could proceed without re-validating the fresh state against Horizon/backend.

Solution

1. Added lockedFields support to SendForm, marking destinationAddress, amount, destinationAssetCode, and amountType as read-only / disabled in PayRequest.
2. Updated PayRequests onSubmit handler to re-fetch latest request state (refetch()) and ensure the request is still open and matching requesterPublicKey, amount, and assetCode before requesting a quote.
3. Added comprehensive unit tests in src/pages/PayRequest.test.tsx verifying field locking and stale/cancelled request blocking.