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-pure/internals/inherit-if-required.js

@@ -0,0 +1,19 @@
+'use strict';
+var isCallable = require('../internals/is-callable');
+var isObject = require('../internals/is-object');
+var setPrototypeOf = require('../internals/object-set-prototype-of');
+
+// makes subclassing work correct for wrapped built-ins
+module.exports = function ($this, dummy, Wrapper) {
+  var NewTarget, NewTargetPrototype;
+  if (
+    // it can work only with native `setPrototypeOf`
+    setPrototypeOf &&
+    // we haven't completely correct pre-ES6 way for getting `new.target`, so use this
+    isCallable(NewTarget = dummy.constructor) &&
+    NewTarget !== Wrapper &&
+    isObject(NewTargetPrototype = NewTarget.prototype) &&
+    NewTargetPrototype !== Wrapper.prototype
+  ) setPrototypeOf($this, NewTargetPrototype);
+  return $this;
+};