Fix paid invoice form validation

main.js

@@ -341,10 +341,15 @@ function syncDocumentTypeControls() {
   const isPersonalPaid = selectedType === "personal_paid";
   setVisibility(paymentTermField, !isPersonalPaid);
   setVisibility(paidDocumentHint, isPersonalPaid);
-  if (isPersonalPaid && invoiceForm?.elements.paymentTerm) {
-    invoiceForm.elements.paymentTerm.value = "0";
-  } else if (invoiceForm?.elements.paymentTerm && !invoiceForm.elements.paymentTerm.value) {
-    invoiceForm.elements.paymentTerm.value = "14";
+  const paymentTermInput = invoiceForm?.elements.paymentTerm;
+  if (!paymentTermInput) {
+    return;
+  }
+  paymentTermInput.disabled = isPersonalPaid;
+  if (isPersonalPaid) {
+    paymentTermInput.value = "";
+  } else if (!paymentTermInput.value) {
+    paymentTermInput.value = "14";
   }
 }
 
@@ -859,7 +864,7 @@ function startInvoiceEdit(invoiceId) {
     syncDocumentTypeControls();
   }
   invoiceForm.elements.saleDate.value = invoice.sale_date || "";
-  invoiceForm.elements.paymentTerm.value = isPaidInvoice(invoice) ? 0 : (invoice.payment_term || 14);
+  invoiceForm.elements.paymentTerm.value = isPaidInvoice(invoice) ? "" : (invoice.payment_term || 14);
 
   if (invoice.client) {
     setClientFormValues(invoice.client);