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.symbol.to-primitive.js

@@ -0,0 +1,11 @@
+'use strict';
+var defineWellKnownSymbol = require('../internals/well-known-symbol-define');
+var defineSymbolToPrimitive = require('../internals/symbol-define-to-primitive');
+
+// `Symbol.toPrimitive` well-known symbol
+// https://tc39.es/ecma262/#sec-symbol.toprimitive
+defineWellKnownSymbol('toPrimitive');
+
+// `Symbol.prototype[@@toPrimitive]` method
+// https://tc39.es/ecma262/#sec-symbol.prototype-@@toprimitive
+defineSymbolToPrimitive();