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/lodash/toPlainObject.js

@@ -0,0 +1,32 @@
+var copyObject = require('./_copyObject'),
+    keysIn = require('./keysIn');
+
+/**
+ * Converts `value` to a plain object flattening inherited enumerable string
+ * keyed properties of `value` to own properties of the plain object.
+ *
+ * @static
+ * @memberOf _
+ * @since 3.0.0
+ * @category Lang
+ * @param {*} value The value to convert.
+ * @returns {Object} Returns the converted plain object.
+ * @example
+ *
+ * function Foo() {
+ *   this.b = 2;
+ * }
+ *
+ * Foo.prototype.c = 3;
+ *
+ * _.assign({ 'a': 1 }, new Foo);
+ * // => { 'a': 1, 'b': 2 }
+ *
+ * _.assign({ 'a': 1 }, _.toPlainObject(new Foo));
+ * // => { 'a': 1, 'b': 2, 'c': 3 }
+ */
+function toPlainObject(value) {
+  return copyObject(value, keysIn(value));
+}
+
+module.exports = toPlainObject;