Nonces, retries and refunds

Credits are debited when the action is created, not when it is signed. So:

• Persist the nonce before prompting for a signature — use onNonce.
• Never re-create an action to retry. That debits again. Poll instead: await turbo.getArNSActionStatus(nonce).
• An abandoned action refunds itself — don't build a refund flow.
• Replaying signArNSAction on a completed action returns { alreadyCompleted: true } rather than buying twice.

InsufficientCreditsError (HTTP 402) is thrown when the balance is short; prompt a top-up, then create a fresh action.