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/isError.js

@@ -0,0 +1,36 @@
+var baseGetTag = require('./_baseGetTag'),
+    isObjectLike = require('./isObjectLike'),
+    isPlainObject = require('./isPlainObject');
+
+/** `Object#toString` result references. */
+var domExcTag = '[object DOMException]',
+    errorTag = '[object Error]';
+
+/**
+ * Checks if `value` is an `Error`, `EvalError`, `RangeError`, `ReferenceError`,
+ * `SyntaxError`, `TypeError`, or `URIError` object.
+ *
+ * @static
+ * @memberOf _
+ * @since 3.0.0
+ * @category Lang
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is an error object, else `false`.
+ * @example
+ *
+ * _.isError(new Error);
+ * // => true
+ *
+ * _.isError(Error);
+ * // => false
+ */
+function isError(value) {
+  if (!isObjectLike(value)) {
+    return false;
+  }
+  var tag = baseGetTag(value);
+  return tag == errorTag || tag == domExcTag ||
+    (typeof value.message == 'string' && typeof value.name == 'string' && !isPlainObject(value));
+}
+
+module.exports = isError;