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/2020/BigInt/unsignedRightShift.js

@@ -0,0 +1,13 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+// https://262.ecma-international.org/11.0/#sec-numeric-types-bigint-unsignedRightShift
+
+module.exports = function BigIntUnsignedRightShift(x, y) {
+	if (typeof x !== 'bigint' || typeof y !== 'bigint') {
+		throw new $TypeError('Assertion failed: `x` and `y` arguments must be BigInts');
+	}
+
+	throw new $TypeError('BigInts have no unsigned right shift, use >> instead');
+};