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/runtime/RelativeUrlRuntimeModule.js

@@ -0,0 +1,45 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+*/
+
+"use strict";
+
+const RuntimeGlobals = require("../RuntimeGlobals");
+const Template = require("../Template");
+const HelperRuntimeModule = require("./HelperRuntimeModule");
+
+/** @typedef {import("../Compilation")} Compilation */
+
+class RelativeUrlRuntimeModule extends HelperRuntimeModule {
+	constructor() {
+		super("relative url");
+	}
+
+	/**
+	 * Generates runtime code for this runtime module.
+	 * @returns {string | null} runtime code
+	 */
+	generate() {
+		const compilation = /** @type {Compilation} */ (this.compilation);
+		const { runtimeTemplate } = compilation;
+		return Template.asString([
+			`${RuntimeGlobals.relativeUrl} = function RelativeURL(url) {`,
+			Template.indent([
+				'var realUrl = new URL(url, "x:/");',
+				"var values = {};",
+				"for (var key in realUrl) values[key] = realUrl[key];",
+				"values.href = url;",
+				'values.pathname = url.replace(/[?#].*/, "");',
+				'values.origin = values.protocol = "";',
+				`values.toString = values.toJSON = ${runtimeTemplate.returningFunction(
+					"url"
+				)};`,
+				"for (var key in values) Object.defineProperty(this, key, { enumerable: true, configurable: true, value: values[key] });"
+			]),
+			"};",
+			`${RuntimeGlobals.relativeUrl}.prototype = URL.prototype;`
+		]);
+	}
+}
+
+module.exports = RelativeUrlRuntimeModule;