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/ModuleRestoreError.js

@@ -0,0 +1,47 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Author Tobias Koppers @sokra
+*/
+
+"use strict";
+
+const WebpackError = require("./WebpackError");
+
+/** @typedef {import("./Module")} Module */
+
+class ModuleRestoreError extends WebpackError {
+	/**
+	 * Creates an instance of ModuleRestoreError.
+	 * @param {Module} module module tied to dependency
+	 * @param {string | Error} err error thrown
+	 */
+	constructor(module, err) {
+		let message = "Module restore failed: ";
+		/** @type {string | undefined} */
+		const details = undefined;
+		if (err !== null && typeof err === "object") {
+			if (typeof err.stack === "string" && err.stack) {
+				const stack = err.stack;
+				message += stack;
+			} else if (typeof err.message === "string" && err.message) {
+				message += err.message;
+			} else {
+				message += err;
+			}
+		} else {
+			message += String(err);
+		}
+
+		super(message);
+
+		/** @type {string} */
+		this.name = "ModuleRestoreError";
+		/** @type {string | undefined} */
+		this.details = details;
+		this.module = module;
+		this.error = err;
+	}
+}
+
+/** @type {typeof ModuleRestoreError} */
+module.exports = ModuleRestoreError;