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/isValidHex.js

@@ -0,0 +1,11 @@
+'use strict';
+
+/**
+ * Check if a value is a valid 3, 4, 6 or 8 digit hex
+ *
+ * @param {string} value
+ * @returns {boolean}
+ */
+module.exports = function isValidHex(value) {
+	return /^#(?:[0-9a-fA-F]{3,4}|[0-9a-fA-F]{6}|[0-9a-fA-F]{8})$/.test(value);
+};