fix: prevent asset conflicts between React and Grid.js versions

Add coexistence checks to all enqueue methods to prevent loading
both React and Grid.js assets simultaneously.

Changes:
- ReactAdmin.php: Only enqueue React assets when ?react=1
- Init.php: Skip Grid.js when React active on admin pages
- Form.php, Coupon.php, Access.php: Restore classic assets when ?react=0
- Customer.php, Product.php, License.php: Add coexistence checks

Now the toggle between Classic and React versions works correctly.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

node_modules/jsx-ast-utils/src/values/expressions/SpreadElement.js

@@ -0,0 +1,11 @@
+/**
+ * Extractor function for a SpreadElement type value node.
+ * We can't statically evaluate an array spread, so just return
+ * undefined.
+ *
+ * @param - value - AST Value object with type `SpreadElement`
+ * @returns - An prototypeless object.
+ */
+export default function extractValueFromSpreadElement() {
+  return undefined;
+}