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/core-js/modules/es.math.tanh.js

@@ -0,0 +1,16 @@
+'use strict';
+var $ = require('../internals/export');
+var expm1 = require('../internals/math-expm1');
+
+var exp = Math.exp;
+
+// `Math.tanh` method
+// https://tc39.es/ecma262/#sec-math.tanh
+$({ target: 'Math', stat: true }, {
+  tanh: function tanh(x) {
+    var n = +x;
+    var a = expm1(n);
+    var b = expm1(-n);
+    return a === Infinity ? 1 : b === Infinity ? -1 : (a - b) / (exp(n) + exp(-n));
+  }
+});