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/HasOwnPropertyRuntimeModule.js

@@ -0,0 +1,36 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Author Sergey Melyukov @smelukov
+*/
+
+"use strict";
+
+const RuntimeGlobals = require("../RuntimeGlobals");
+const RuntimeModule = require("../RuntimeModule");
+const Template = require("../Template");
+
+/** @typedef {import("../Compilation")} Compilation */
+
+class HasOwnPropertyRuntimeModule extends RuntimeModule {
+	constructor() {
+		super("hasOwnProperty shorthand");
+	}
+
+	/**
+	 * 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.hasOwnProperty} = ${runtimeTemplate.returningFunction(
+				"Object.prototype.hasOwnProperty.call(obj, prop)",
+				"obj, prop"
+			)}`
+		]);
+	}
+}
+
+module.exports = HasOwnPropertyRuntimeModule;