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/@sinonjs/commons/lib/value-to-string.test.js

@@ -0,0 +1,20 @@
+"use strict";
+
+var assert = require("@sinonjs/referee-sinon").assert;
+var valueToString = require("./value-to-string");
+
+describe("util/core/valueToString", function () {
+    it("returns string representation of an object", function () {
+        var obj = {};
+
+        assert.equals(valueToString(obj), obj.toString());
+    });
+
+    it("returns 'null' for literal null'", function () {
+        assert.equals(valueToString(null), "null");
+    });
+
+    it("returns 'undefined' for literal undefined", function () {
+        assert.equals(valueToString(undefined), "undefined");
+    });
+});