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/_stackHas.js

@@ -0,0 +1,14 @@
+/**
+ * Checks if a stack value for `key` exists.
+ *
+ * @private
+ * @name has
+ * @memberOf Stack
+ * @param {string} key The key of the entry to check.
+ * @returns {boolean} Returns `true` if an entry for `key` exists, else `false`.
+ */
+function stackHas(key) {
+  return this.__data__.has(key);
+}
+
+module.exports = stackHas;