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/errors/BuildCycleError.js

@@ -0,0 +1,30 @@
+/*
+	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 BuildCycleError extends WebpackError {
+	/**
+	 * Creates an instance of BuildCycleError.
+	 * @param {Module} module the module starting the cycle
+	 */
+	constructor(module) {
+		super(
+			"There is a circular build dependency, which makes it impossible to create this module"
+		);
+
+		/** @type {string} */
+		this.name = "BuildCycleError";
+		/** @type {Module} */
+		this.module = module;
+	}
+}
+
+/** @type {typeof BuildCycleError} */
+module.exports = BuildCycleError;