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/axios/lib/helpers/isAbsoluteURL.js

@@ -0,0 +1,19 @@
+'use strict';
+
+/**
+ * Determines whether the specified URL is absolute
+ *
+ * @param {string} url The URL to test
+ *
+ * @returns {boolean} True if the specified URL is absolute, otherwise false
+ */
+export default function isAbsoluteURL(url) {
+  // A URL is considered absolute if it begins with "<scheme>://" or "//" (protocol-relative URL).
+  // RFC 3986 defines scheme name as a sequence of characters beginning with a letter and followed
+  // by any combination of letters, digits, plus, period, or hyphen.
+  if (typeof url !== 'string') {
+    return false;
+  }
+
+  return /^([a-z][a-z\d+\-.]*:)?\/\//i.test(url);
+}