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/string-trim-end.js

@@ -0,0 +1,11 @@
+'use strict';
+var $trimEnd = require('../internals/string-trim').end;
+var forcedStringTrimMethod = require('../internals/string-trim-forced');
+
+// `String.prototype.{ trimEnd, trimRight }` method
+// https://tc39.es/ecma262/#sec-string.prototype.trimend
+// https://tc39.es/ecma262/#String.prototype.trimright
+module.exports = forcedStringTrimMethod('trimEnd') ? function trimEnd() {
+  return $trimEnd(this);
+// eslint-disable-next-line es/no-string-prototype-trimstart-trimend -- safe
+} : ''.trimEnd;