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/json-stable-stringify-without-jsonify/test/to-json.js

@@ -0,0 +1,20 @@
+var test = require('tape');
+var stringify = require('../');
+
+test('toJSON function', function (t) {
+    t.plan(1);
+    var obj = { one: 1, two: 2, toJSON: function() { return { one: 1 }; } };
+    t.equal(stringify(obj), '{"one":1}' );
+});
+
+test('toJSON returns string', function (t) {
+	t.plan(1);
+	var obj = { one: 1, two: 2, toJSON: function() { return 'one'; } };
+	t.equal(stringify(obj), '"one"');
+});
+
+test('toJSON returns array', function (t) {
+	t.plan(1);
+	var obj = { one: 1, two: 2, toJSON: function() { return ['one']; } };
+	t.equal(stringify(obj), '["one"]');
+});