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/2016/ToLength.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var MAX_SAFE_INTEGER = require('math-intrinsics/constants/maxSafeInteger');
+
+var ToInteger = require('./ToInteger');
+
+// https://262.ecma-international.org/6.0/#sec-tolength
+
+module.exports = function ToLength(argument) {
+	var len = ToInteger(argument);
+	if (len <= 0) { return 0; } // includes converting -0 to +0
+	if (len > MAX_SAFE_INTEGER) { return MAX_SAFE_INTEGER; }
+	return len;
+};