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/array-flatten/array-flatten.js

@@ -0,0 +1,64 @@
+'use strict'
+
+/**
+ * Expose `arrayFlatten`.
+ */
+module.exports = arrayFlatten
+
+/**
+ * Recursive flatten function with depth.
+ *
+ * @param  {Array}  array
+ * @param  {Array}  result
+ * @param  {Number} depth
+ * @return {Array}
+ */
+function flattenWithDepth (array, result, depth) {
+  for (var i = 0; i < array.length; i++) {
+    var value = array[i]
+
+    if (depth > 0 && Array.isArray(value)) {
+      flattenWithDepth(value, result, depth - 1)
+    } else {
+      result.push(value)
+    }
+  }
+
+  return result
+}
+
+/**
+ * Recursive flatten function. Omitting depth is slightly faster.
+ *
+ * @param  {Array} array
+ * @param  {Array} result
+ * @return {Array}
+ */
+function flattenForever (array, result) {
+  for (var i = 0; i < array.length; i++) {
+    var value = array[i]
+
+    if (Array.isArray(value)) {
+      flattenForever(value, result)
+    } else {
+      result.push(value)
+    }
+  }
+
+  return result
+}
+
+/**
+ * Flatten an array, with the ability to define a depth.
+ *
+ * @param  {Array}  array
+ * @param  {Number} depth
+ * @return {Array}
+ */
+function arrayFlatten (array, depth) {
+  if (depth == null) {
+    return flattenForever(array, [])
+  }
+
+  return flattenWithDepth(array, [], depth)
+}