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/global.test.js

@@ -0,0 +1,16 @@
+"use strict";
+
+var assert = require("@sinonjs/referee-sinon").assert;
+var globalObject = require("./global");
+
+describe("global", function () {
+    before(function () {
+        if (typeof global === "undefined") {
+            this.skip();
+        }
+    });
+
+    it("is same as global", function () {
+        assert.same(globalObject, global);
+    });
+});