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/UnhandledSchemeError.js

@@ -0,0 +1,40 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Author Ivan Kopeykin @vankop
+*/
+
+"use strict";
+
+const WebpackError = require("./WebpackError");
+const makeSerializable = require("./util/makeSerializable");
+
+/**
+ * Error raised when webpack encounters a resource URI scheme that no installed
+ * plugin knows how to read.
+ */
+class UnhandledSchemeError extends WebpackError {
+	/**
+	 * Creates an error explaining that the current resource scheme is not
+	 * supported by the active plugin set.
+	 * @param {string} scheme scheme
+	 * @param {string} resource resource
+	 */
+	constructor(scheme, resource) {
+		super(
+			`Reading from "${resource}" is not handled by plugins (Unhandled scheme).` +
+				'\nWebpack supports "data:" and "file:" URIs by default.' +
+				`\nYou may need an additional plugin to handle "${scheme}:" URIs.`
+		);
+		this.file = resource;
+		/** @type {string} */
+		this.name = "UnhandledSchemeError";
+	}
+}
+
+makeSerializable(
+	UnhandledSchemeError,
+	"webpack/lib/UnhandledSchemeError",
+	"UnhandledSchemeError"
+);
+
+module.exports = UnhandledSchemeError;