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-weakmap/index.js

@@ -0,0 +1,46 @@
+'use strict';
+
+var $WeakMap = typeof WeakMap === 'function' && WeakMap.prototype ? WeakMap : null;
+var $WeakSet = typeof WeakSet === 'function' && WeakSet.prototype ? WeakSet : null;
+
+var exported;
+
+if (!$WeakMap) {
+	/** @type {import('.')} */
+	// eslint-disable-next-line no-unused-vars
+	exported = function isWeakMap(x) {
+		// `WeakMap` is not present in this environment.
+		return false;
+	};
+}
+
+var $mapHas = $WeakMap ? $WeakMap.prototype.has : null;
+var $setHas = $WeakSet ? $WeakSet.prototype.has : null;
+if (!exported && !$mapHas) {
+	/** @type {import('.')} */
+	// eslint-disable-next-line no-unused-vars
+	exported = function isWeakMap(x) {
+		// `WeakMap` does not have a `has` method
+		return false;
+	};
+}
+
+/** @type {import('.')} */
+module.exports = exported || function isWeakMap(x) {
+	if (!x || typeof x !== 'object') {
+		return false;
+	}
+	try {
+		$mapHas.call(x, $mapHas);
+		if ($setHas) {
+			try {
+				$setHas.call(x, $setHas);
+			} catch (e) {
+				return true;
+			}
+		}
+		// @ts-expect-error TS can't figure out that $WeakMap is always truthy here
+		return x instanceof $WeakMap; // core-js workaround, pre-v3
+	} catch (e) {}
+	return false;
+};