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/2023/GetStringIndex.js

@@ -0,0 +1,27 @@
+'use strict';
+
+var callBound = require('call-bound');
+var $TypeError = require('es-errors/type');
+var isInteger = require('math-intrinsics/isInteger');
+
+var StringToCodePoints = require('./StringToCodePoints');
+
+var $indexOf = callBound('String.prototype.indexOf');
+
+// https://262.ecma-international.org/13.0/#sec-getstringindex
+
+module.exports = function GetStringIndex(S, e) {
+	if (typeof S !== 'string') {
+		throw new $TypeError('Assertion failed: `S` must be a String');
+	}
+	if (!isInteger(e) || e < 0) {
+		throw new $TypeError('Assertion failed: `e` must be a non-negative integer');
+	}
+
+	if (S === '') {
+		return 0;
+	}
+	var codepoints = StringToCodePoints(S);
+	var eUTF = e >= codepoints.length ? S.length : $indexOf(S, codepoints[e]);
+	return eUTF;
+};