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/PlatformPlugin.js

@@ -0,0 +1,42 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Authors Ivan Kopeykin @vankop
+*/
+
+"use strict";
+
+/** @typedef {import("./Compiler")} Compiler */
+/** @typedef {import("./config/target").PlatformTargetProperties} PlatformTargetProperties */
+
+const PLUGIN_NAME = "PlatformPlugin";
+
+/**
+ * Should be used only for "target === false" or
+ * when you want to overwrite platform target properties
+ */
+class PlatformPlugin {
+	/**
+	 * Creates an instance of PlatformPlugin.
+	 * @param {Partial<PlatformTargetProperties>} platform target properties
+	 */
+	constructor(platform) {
+		/** @type {Partial<PlatformTargetProperties>} */
+		this.platform = platform;
+	}
+
+	/**
+	 * Applies the plugin by registering its hooks on the compiler.
+	 * @param {Compiler} compiler the compiler instance
+	 * @returns {void}
+	 */
+	apply(compiler) {
+		compiler.hooks.environment.tap(PLUGIN_NAME, () => {
+			compiler.platform = {
+				...compiler.platform,
+				...this.platform
+			};
+		});
+	}
+}
+
+module.exports = PlatformPlugin;