fix(api): Azure revoke quote endpoint returns 0/"" when Azure omits the refund amount (misleading dialog) #447

From the review of #445 (closes #96). extractAzureRefundQuote (internal/api/handler_purchases_revoke.go ~529-543) fills 0/"" when Azure omits BillingRefundAmount, so the confirm dialog says 'Azure will refund 0.00 .' and the POST then gets a 400 (it fails closed, but the text is misleading). Return 422 from calculateAzureRevoke when amount or currency is missing, and reuse that one extractor in azureCalculateRefund (the parsing is duplicated today). This also removes the frontend nit at history.ts ~1432, where a null refund_amount throws before the dialog opens.