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/@wordpress/icons/build-module/library/stack.js

@@ -0,0 +1,13 @@
+import { createElement } from "react";
+/**
+ * WordPress dependencies
+ */
+import { SVG, Path } from '@wordpress/primitives';
+const stack = createElement(SVG, {
+  xmlns: "http://www.w3.org/2000/svg",
+  viewBox: "0 0 24 24"
+}, createElement(Path, {
+  d: "M17.5 4v5a2 2 0 0 1-2 2h-7a2 2 0 0 1-2-2V4H8v5a.5.5 0 0 0 .5.5h7A.5.5 0 0 0 16 9V4h1.5Zm0 16v-5a2 2 0 0 0-2-2h-7a2 2 0 0 0-2 2v5H8v-5a.5.5 0 0 1 .5-.5h7a.5.5 0 0 1 .5.5v5h1.5Z"
+}));
+export default stack;
+//# sourceMappingURL=stack.js.map