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/internals/set-species.js

@@ -0,0 +1,18 @@
+'use strict';
+var getBuiltIn = require('../internals/get-built-in');
+var defineBuiltInAccessor = require('../internals/define-built-in-accessor');
+var wellKnownSymbol = require('../internals/well-known-symbol');
+var DESCRIPTORS = require('../internals/descriptors');
+
+var SPECIES = wellKnownSymbol('species');
+
+module.exports = function (CONSTRUCTOR_NAME) {
+  var Constructor = getBuiltIn(CONSTRUCTOR_NAME);
+
+  if (DESCRIPTORS && Constructor && !Constructor[SPECIES]) {
+    defineBuiltInAccessor(Constructor, SPECIES, {
+      configurable: true,
+      get: function () { return this; }
+    });
+  }
+};