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/istanbul-lib-source-maps/lib/pathutils.js

@@ -0,0 +1,21 @@
+/*
+ Copyright 2015, Yahoo Inc.
+ Copyrights licensed under the New BSD License. See the accompanying LICENSE file for terms.
+ */
+'use strict';
+
+const path = require('path');
+
+module.exports = {
+    isAbsolute: path.isAbsolute,
+    asAbsolute(file, baseDir) {
+        return path.isAbsolute(file)
+            ? file
+            : path.resolve(baseDir || process.cwd(), file);
+    },
+    relativeTo(file, origFile) {
+        return path.isAbsolute(file)
+            ? file
+            : path.resolve(path.dirname(origFile), file);
+    }
+};