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/get-mode-option.js

@@ -0,0 +1,8 @@
+'use strict';
+var $TypeError = TypeError;
+
+module.exports = function (options) {
+  var mode = options && options.mode;
+  if (mode === undefined || mode === 'shortest' || mode === 'longest' || mode === 'strict') return mode || 'shortest';
+  throw new $TypeError('Incorrect `mode` option');
+};