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/is-weakref/test/index.js

@@ -0,0 +1,26 @@
+'use strict';
+
+var test = require('tape');
+var inspect = require('object-inspect');
+var forEach = require('for-each');
+
+var isWeakRef = require('..');
+
+test('isWeakRef', function (t) {
+	t.equal(typeof isWeakRef, 'function', 'is a function');
+
+	var nonWeakRefs = [undefined, null, true, false, 42, 0, Infinity, NaN, /a/g, function () {}, {}, []];
+	forEach(nonWeakRefs, function (nonWeakRef) {
+		t.equal(isWeakRef(nonWeakRef), false, inspect(nonWeakRef) + ' is not a WeakRef');
+	});
+
+	t.test('actual WeakRefs', { skip: typeof WeakRef === 'undefined' }, function (st) {
+		var ref = new WeakRef({});
+
+		st.equal(isWeakRef(ref), true, inspect(ref) + ' is a WeakRef');
+
+		st.end();
+	});
+
+	t.end();
+});