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/AbstractMethodError.js

@@ -0,0 +1,65 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Author Ivan Kopeykin @vankop
+*/
+
+"use strict";
+
+const WebpackError = require("./WebpackError");
+
+const CURRENT_METHOD_REGEXP = /at ([a-zA-Z0-9_.]*)/;
+
+/**
+ * Creates the error message shown when an abstract API is called without
+ * being implemented by a subclass.
+ * @param {string=} method method name
+ * @returns {string} message
+ */
+function createMessage(method) {
+	return `Abstract method${method ? ` ${method}` : ""}. Must be overridden.`;
+}
+
+/**
+ * Captures a stack trace so the calling method name can be folded into the
+ * final abstract-method error message.
+ * @constructor
+ */
+function Message() {
+	/** @type {string | undefined} */
+	this.stack = undefined;
+	Error.captureStackTrace(this);
+	/** @type {RegExpMatchArray | null} */
+	const match =
+		/** @type {string} */
+		(/** @type {unknown} */ (this.stack))
+			.split("\n")[3]
+			.match(CURRENT_METHOD_REGEXP);
+
+	this.message = match && match[1] ? createMessage(match[1]) : createMessage();
+}
+
+/**
+ * Error thrown when code reaches a method that is intended to be overridden by
+ * a subclass.
+ * @example
+ * ```js
+ * class FooClass {
+ *     abstractMethod() {
+ *         throw new AbstractMethodError(); // error message: Abstract method FooClass.abstractMethod. Must be overridden.
+ *     }
+ * }
+ * ```
+ */
+class AbstractMethodError extends WebpackError {
+	/**
+	 * Creates an error whose message points at the abstract method that was
+	 * invoked.
+	 */
+	constructor() {
+		super(new Message().message);
+		/** @type {string} */
+		this.name = "AbstractMethodError";
+	}
+}
+
+module.exports = AbstractMethodError;