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/valtio/utils/proxySet.d.ts

@@ -0,0 +1,16 @@
+/**
+ * proxySet
+ *
+ * This is to create a proxy which mimic the native Set behavior.
+ * The API is the same as Set API
+ *
+ * @example
+ * import { proxySet } from 'valtio/utils'
+ * const state = proxySet([1,2,3])
+ * //can be used inside a proxy as well
+ * const state = proxy({
+ *   count: 1,
+ *   set: proxySet()
+ * })
+ */
+export declare function proxySet<T>(initialValues?: Iterable<T> | null): Set<T>;