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/index.test.js

@@ -0,0 +1,31 @@
+"use strict";
+
+var assert = require("@sinonjs/referee-sinon").assert;
+var index = require("./index");
+
+var expectedMethods = [
+    "calledInOrder",
+    "className",
+    "every",
+    "functionName",
+    "orderByFirstCall",
+    "typeOf",
+    "valueToString",
+];
+var expectedObjectProperties = ["deprecated", "prototypes"];
+
+describe("package", function () {
+    // eslint-disable-next-line mocha/no-setup-in-describe
+    expectedMethods.forEach(function (name) {
+        it(`should export a method named ${name}`, function () {
+            assert.isFunction(index[name]);
+        });
+    });
+
+    // eslint-disable-next-line mocha/no-setup-in-describe
+    expectedObjectProperties.forEach(function (name) {
+        it(`should export an object property named ${name}`, function () {
+            assert.isObject(index[name]);
+        });
+    });
+});