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/well-known-symbol-define.js

@@ -0,0 +1,12 @@
+'use strict';
+var path = require('../internals/path');
+var hasOwn = require('../internals/has-own-property');
+var wrappedWellKnownSymbolModule = require('../internals/well-known-symbol-wrapped');
+var defineProperty = require('../internals/object-define-property').f;
+
+module.exports = function (NAME) {
+  var Symbol = path.Symbol || (path.Symbol = {});
+  if (!hasOwn(Symbol, NAME)) defineProperty(Symbol, NAME, {
+    value: wrappedWellKnownSymbolModule.f(NAME)
+  });
+};