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/GlobalRuntimeModule.js

@@ -0,0 +1,48 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+*/
+
+"use strict";
+
+const RuntimeGlobals = require("../RuntimeGlobals");
+const RuntimeModule = require("../RuntimeModule");
+const Template = require("../Template");
+
+class GlobalRuntimeModule extends RuntimeModule {
+	constructor() {
+		super("global");
+	}
+
+	/**
+	 * Generates runtime code for this runtime module.
+	 * @returns {string | null} runtime code
+	 */
+	generate() {
+		return Template.asString([
+			`${RuntimeGlobals.global} = (function() {`,
+			Template.indent([
+				"if (typeof globalThis === 'object') return globalThis;",
+				"try {",
+				Template.indent(
+					// This works in non-strict mode
+					// or
+					// This works if eval is allowed (see CSP)
+					"return this || new Function('return this')();"
+				),
+				"} catch (e) {",
+				Template.indent(
+					// This works if the window reference is available
+					"if (typeof window === 'object') return window;"
+				),
+				"}"
+				// It can still be `undefined`, but nothing to do about it...
+				// We return `undefined`, instead of nothing here, so it's
+				// easier to handle this case:
+				//   if (!global) { … }
+			]),
+			"})();"
+		]);
+	}
+}
+
+module.exports = GlobalRuntimeModule;