[UI] Fix serial and batch code generators (#9772)

* Tweak stock item form

- Fix batch code placeholder
- Fix serial number placeholder

* Tweak build output form

* More cleanup

* Fix for PurchaseOrderForm

* Refactoring placeholder values

src/frontend/tests/pages/pui_build.spec.ts

@@ -167,6 +167,8 @@ test('Build Order - Build Outputs', async ({ browser }) => {
     .getByLabel('text-field-serial_numbers')
     .getAttribute('placeholder');
 
+  expect(placeholder).toContain('Next serial number');
+
   let sn = 1;
 
   if (!!placeholder && placeholder.includes('Next serial number')) {