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/2015/StringGetIndexProperty.js

@@ -0,0 +1,50 @@
+'use strict';
+
+var callBound = require('call-bound');
+
+var $TypeError = require('es-errors/type');
+
+var $charAt = callBound('String.prototype.charAt');
+
+var isString = require('is-string');
+var isNegativeZero = require('math-intrinsics/isNegativeZero');
+var unbox = require('unbox-primitive');
+
+var CanonicalNumericIndexString = require('./CanonicalNumericIndexString');
+var IsInteger = require('./IsInteger');
+var isPropertyKey = require('../helpers/isPropertyKey');
+
+// https://262.ecma-international.org/6.0/#sec-stringgetindexproperty
+
+module.exports = function StringGetIndexProperty(S, P) {
+	if (typeof S === 'string' || !isString(S)) {
+		throw new $TypeError('Assertion failed: `S` must be a boxed String Object');
+	}
+	if (!isPropertyKey(P)) {
+		throw new $TypeError('Assertion failed: `P` must be a Property Key');
+	}
+
+	if (typeof P !== 'string') {
+		return void undefined;
+	}
+
+	var index = CanonicalNumericIndexString(P);
+	if (typeof index === 'undefined' || !IsInteger(index) || isNegativeZero(index)) {
+		return void undefined;
+	}
+
+	var str = unbox(S);
+	var len = str.length;
+	if (index < 0 || len <= index) {
+		return void undefined;
+	}
+
+	var resultStr = $charAt(str, index);
+
+	return {
+		'[[Configurable]]': false,
+		'[[Enumerable]]': true,
+		'[[Value]]': resultStr,
+		'[[Writable]]': false
+	};
+};