Add configurable author price calculator

scripts/author_price.js

@@ -0,0 +1,12 @@
+(() => {
+  const checkbox = document.querySelector('#use-author-margin');
+  const discountInput = document.querySelector('#discount-percent');
+  if (!checkbox || !discountInput) return;
+
+  const updateDiscountInput = () => {
+    discountInput.disabled = checkbox.checked;
+  };
+
+  checkbox.addEventListener('change', updateDiscountInput);
+  updateDiscountInput();
+})();