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/array-includes/test/index.js

@@ -0,0 +1,17 @@
+'use strict';
+
+var includes = require('../');
+var test = require('tape');
+var runTests = require('./tests');
+
+test('as a function', function (t) {
+	t.test('bad array/this value', function (st) {
+		st['throws'](function () { includes(undefined, 'a'); }, TypeError, 'undefined is not an object');
+		st['throws'](function () { includes(null, 'a'); }, TypeError, 'null is not an object');
+		st.end();
+	});
+
+	runTests(includes, t);
+
+	t.end();
+});