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/resolve-dir/index.js

@@ -0,0 +1,22 @@
+/*!
+ * resolve-dir <https://github.com/jonschlinkert/resolve-dir>
+ *
+ * Copyright (c) 2015, Jon Schlinkert.
+ * Licensed under the MIT License.
+ */
+
+'use strict';
+
+var path = require('path');
+var expand = require('expand-tilde');
+var gm = require('global-modules');
+
+module.exports = function resolveDir(dir) {
+  if (dir.charAt(0) === '~') {
+    dir = expand(dir);
+  }
+  if (dir.charAt(0) === '@') {
+    dir = path.join(gm, dir.slice(1));
+  }
+  return dir;
+};