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/stylelint/lib/utils/isCustomFunction.js

@@ -0,0 +1,11 @@
+'use strict';
+
+/**
+ * Check whether a function is custom / user-defined
+ * https://github.com/w3c/css-houdini-drafts/issues/1007
+ * @param {string} func
+ * @returns {boolean}
+ */
+module.exports = function isCustomFunction(func) {
+	return func.startsWith('--');
+};