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.function.has-instance.js

@@ -0,0 +1,20 @@
+'use strict';
+var isCallable = require('../internals/is-callable');
+var isObject = require('../internals/is-object');
+var definePropertyModule = require('../internals/object-define-property');
+var isPrototypeOf = require('../internals/object-is-prototype-of');
+var wellKnownSymbol = require('../internals/well-known-symbol');
+var makeBuiltIn = require('../internals/make-built-in');
+
+var HAS_INSTANCE = wellKnownSymbol('hasInstance');
+var FunctionPrototype = Function.prototype;
+
+// `Function.prototype[@@hasInstance]` method
+// https://tc39.es/ecma262/#sec-function.prototype-@@hasinstance
+if (!(HAS_INSTANCE in FunctionPrototype)) {
+  definePropertyModule.f(FunctionPrototype, HAS_INSTANCE, { value: makeBuiltIn(function (O) {
+    if (!isCallable(this) || !isObject(O)) return false;
+    var P = this.prototype;
+    return isObject(P) ? isPrototypeOf(P, O) : O instanceof this;
+  }, HAS_INSTANCE) });
+}