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/webpack/lib/NodeStuffInWebError.js

@@ -0,0 +1,36 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Author Ivan Kopeykin @vankop
+*/
+
+"use strict";
+
+const WebpackError = require("./WebpackError");
+const makeSerializable = require("./util/makeSerializable");
+
+/** @typedef {import("./Dependency").DependencyLocation} DependencyLocation */
+
+class NodeStuffInWebError extends WebpackError {
+	/**
+	 * Creates an instance of NodeStuffInWebError.
+	 * @param {DependencyLocation} loc loc
+	 * @param {string} expression expression
+	 * @param {string} description description
+	 */
+	constructor(loc, expression, description) {
+		super(
+			`${JSON.stringify(
+				expression
+			)} has been used, it will be undefined in next major version.
+${description}`
+		);
+
+		/** @type {string} */
+		this.name = "NodeStuffInWebError";
+		this.loc = loc;
+	}
+}
+
+makeSerializable(NodeStuffInWebError, "webpack/lib/NodeStuffInWebError");
+
+module.exports = NodeStuffInWebError;