fix: certificate button only on paid Purchase Invoices

The certificate is issued when the tax is withheld, i.e. on payment;
show the button once the submitted invoice's status is Paid.

README.md

@@ -22,7 +22,7 @@ without a Thai system font. Switch a user or the print language to `th` to use i
 
 ### Withholding tax certificate
 
-A Purchase Invoice with *Apply Tax Withholding Amount* gets a **Withholding Tax Certificate**
+A paid Purchase Invoice with *Apply Tax Withholding Amount* gets a **Withholding Tax Certificate**
 button that downloads the Revenue Department's Sec. 50 bis form (หนังสือรับรองการหักภาษี ณ ที่จ่าย,
 two copies) as a PDF: the *Withholding Tax Certificate* print format lays the invoice's data
 over the official form (`public/images`). The row the payment is reported on comes from