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/ConcurrentCompilationError.js

@@ -0,0 +1,18 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Author Maksim Nazarjev @acupofspirt
+*/
+
+"use strict";
+
+const WebpackError = require("./WebpackError");
+
+module.exports = class ConcurrentCompilationError extends WebpackError {
+	constructor() {
+		super(
+			"You ran Webpack twice. Each instance only supports a single concurrent compilation at a time."
+		);
+
+		this.name = "ConcurrentCompilationError";
+	}
+};