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/loader-runner/lib/LoaderLoadingError.js

@@ -0,0 +1,13 @@
+"use strict";
+
+class LoadingLoaderError extends Error {
+	constructor(message) {
+		super(message);
+		this.name = "LoaderRunnerError";
+		// For old Node.js engines remove it then we drop them support
+		// eslint-disable-next-line unicorn/no-useless-error-capture-stack-trace
+		Error.captureStackTrace(this, this.constructor);
+	}
+}
+
+module.exports = LoadingLoaderError;