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/IsStringPrefix.js

@@ -0,0 +1,43 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var isPrefixOf = require('../helpers/isPrefixOf');
+
+// var callBound = require('call-bound');
+
+// var $charAt = callBound('String.prototype.charAt');
+
+// https://262.ecma-international.org/9.0/#sec-isstringprefix
+
+module.exports = function IsStringPrefix(p, q) {
+	if (typeof p !== 'string') {
+		throw new $TypeError('Assertion failed: "p" must be a String');
+	}
+
+	if (typeof q !== 'string') {
+		throw new $TypeError('Assertion failed: "q" must be a String');
+	}
+
+	return isPrefixOf(p, q);
+	/*
+	if (p === q || p === '') {
+		return true;
+	}
+
+	var pLength = p.length;
+	var qLength = q.length;
+	if (pLength >= qLength) {
+		return false;
+	}
+
+	// assert: pLength < qLength
+
+	for (var i = 0; i < pLength; i += 1) {
+		if ($charAt(p, i) !== $charAt(q, i)) {
+			return false;
+		}
+	}
+	return true;
+	*/
+};