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/es-abstract/2018/SetFunctionLength.js

@@ -0,0 +1,28 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+var isInteger = require('math-intrinsics/isInteger');
+
+var DefinePropertyOrThrow = require('./DefinePropertyOrThrow');
+var HasOwnProperty = require('./HasOwnProperty');
+var IsExtensible = require('./IsExtensible');
+
+// https://262.ecma-international.org/9.0/#sec-setfunctionlength
+
+module.exports = function SetFunctionLength(F, length) {
+	if (typeof F !== 'function' || !IsExtensible(F) || HasOwnProperty(F, 'length')) {
+		throw new $TypeError('Assertion failed: `F` must be an extensible function and lack an own `length` property');
+	}
+	if (typeof length !== 'number') {
+		throw new $TypeError('Assertion failed: `length` must be a Number');
+	}
+	if (length < 0 || !isInteger(length)) {
+		throw new $TypeError('Assertion failed: `length` must be an integer >= 0');
+	}
+	return DefinePropertyOrThrow(F, 'length', {
+		'[[Configurable]]': true,
+		'[[Enumerable]]': false,
+		'[[Value]]': length,
+		'[[Writable]]': false
+	});
+};