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/lodash/defaultTo.js

@@ -0,0 +1,25 @@
+/**
+ * Checks `value` to determine whether a default value should be returned in
+ * its place. The `defaultValue` is returned if `value` is `NaN`, `null`,
+ * or `undefined`.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.14.0
+ * @category Util
+ * @param {*} value The value to check.
+ * @param {*} defaultValue The default value.
+ * @returns {*} Returns the resolved value.
+ * @example
+ *
+ * _.defaultTo(1, 10);
+ * // => 1
+ *
+ * _.defaultTo(undefined, 10);
+ * // => 10
+ */
+function defaultTo(value, defaultValue) {
+  return (value == null || value !== value) ? defaultValue : value;
+}
+
+module.exports = defaultTo;