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/ToInt16.js

@@ -0,0 +1,10 @@
+'use strict';
+
+var ToUint16 = require('./ToUint16');
+
+// https://262.ecma-international.org/6.0/#sec-toint16
+
+module.exports = function ToInt16(argument) {
+	var int16bit = ToUint16(argument);
+	return int16bit >= 0x8000 ? int16bit - 0x10000 : int16bit;
+};