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-pure/internals/a-map.js

@@ -0,0 +1,10 @@
+'use strict';
+var tryToString = require('../internals/try-to-string');
+
+var $TypeError = TypeError;
+
+// Perform ? RequireInternalSlot(M, [[MapData]])
+module.exports = function (it) {
+  if (typeof it == 'object' && 'size' in it && 'has' in it && 'get' in it && 'set' in it && 'delete' in it && 'entries' in it) return it;
+  throw new $TypeError(tryToString(it) + ' is not a map');
+};