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/is-string/index.js

@@ -0,0 +1,31 @@
+'use strict';
+
+var callBound = require('call-bound');
+
+/** @type {(receiver: ThisParameterType<typeof String.prototype.valueOf>, ...args: Parameters<typeof String.prototype.valueOf>) => ReturnType<typeof String.prototype.valueOf>} */
+var $strValueOf = callBound('String.prototype.valueOf');
+
+/** @type {import('.')} */
+var tryStringObject = function tryStringObject(value) {
+	try {
+		$strValueOf(value);
+		return true;
+	} catch (e) {
+		return false;
+	}
+};
+/** @type {(receiver: ThisParameterType<typeof Object.prototype.toString>, ...args: Parameters<typeof Object.prototype.toString>) => ReturnType<typeof Object.prototype.toString>} */
+var $toString = callBound('Object.prototype.toString');
+var strClass = '[object String]';
+var hasToStringTag = require('has-tostringtag/shams')();
+
+/** @type {import('.')} */
+module.exports = function isString(value) {
+	if (typeof value === 'string') {
+		return true;
+	}
+	if (!value || typeof value !== 'object') {
+		return false;
+	}
+	return hasToStringTag ? tryStringObject(value) : $toString(value) === strClass;
+};