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/symbol-define-to-primitive.js

@@ -0,0 +1,21 @@
+'use strict';
+var call = require('../internals/function-call');
+var getBuiltIn = require('../internals/get-built-in');
+var wellKnownSymbol = require('../internals/well-known-symbol');
+var defineBuiltIn = require('../internals/define-built-in');
+
+module.exports = function () {
+  var Symbol = getBuiltIn('Symbol');
+  var SymbolPrototype = Symbol && Symbol.prototype;
+  var valueOf = SymbolPrototype && SymbolPrototype.valueOf;
+  var TO_PRIMITIVE = wellKnownSymbol('toPrimitive');
+
+  if (SymbolPrototype && !SymbolPrototype[TO_PRIMITIVE]) {
+    // `Symbol.prototype[@@toPrimitive]` method
+    // https://tc39.es/ecma262/#sec-symbol.prototype-@@toprimitive
+    // eslint-disable-next-line no-unused-vars -- required for .length
+    defineBuiltIn(SymbolPrototype, TO_PRIMITIVE, function (hint) {
+      return call(valueOf, this);
+    }, { arity: 1 });
+  }
+};