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-async-function/index.js

@@ -0,0 +1,32 @@
+'use strict';
+
+var callBound = require('call-bound');
+var safeRegexTest = require('safe-regex-test');
+
+var toStr = callBound('Object.prototype.toString');
+var fnToStr = callBound('Function.prototype.toString');
+var isFnRegex = safeRegexTest(/^\s*async(?:\s+function(?:\s+|\()|\s*\()/);
+
+var hasToStringTag = require('has-tostringtag/shams')();
+var getProto = require('get-proto');
+
+var getAsyncFunc = require('async-function');
+
+/** @type {import('.')} */
+module.exports = function isAsyncFunction(fn) {
+	if (typeof fn !== 'function') {
+		return false;
+	}
+	if (isFnRegex(fnToStr(fn))) {
+		return true;
+	}
+	if (!hasToStringTag) {
+		var str = toStr(fn);
+		return str === '[object AsyncFunction]';
+	}
+	if (!getProto) {
+		return false;
+	}
+	var asyncFunc = getAsyncFunc();
+	return asyncFunc && asyncFunc.prototype === getProto(fn);
+};