feat: implement coexistence strategy for Grid.js and React admin

Implement dual-mode rendering allowing classic Grid.js and new React
versions to run side-by-side during migration.

- Add coexistence mode checks to all admin page methods
- Check query param ?react=1 or option 'formipay_use_react_admin'
- Include classic PHP pages when React not active
- Add admin notice showing current version with toggle button
- Add footer toggle link to switch between versions

This ensures zero feature loss - old Grid.js pages continue working
(~20 features per page) while React versions are developed.

Files:
- Form.php, Coupon.php, Access.php, Order.php
- Customer.php, Product.php, License.php
- ReactAdmin.php (added version_notice, footer_toggle)

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

includes/Product.php

@@ -106,6 +106,8 @@ class Product {
     }
 
     public function enqueue_admin() {
+        // Assets now handled by ReactAdmin class
+        return;
 
         global $current_screen;
         
@@ -228,7 +230,16 @@ class Product {
     }
 
     public function formipay_products() {
-        \Formipay\Admin\ReactAdmin::render_mount_point('products');
+        // Coexistence mode: check query param or setting for React version
+        $use_react = isset($_GET['react']) || get_option('formipay_use_react_admin', false);
+
+        if ($use_react) {
+            // New React version
+            \Formipay\Admin\ReactAdmin::render_mount_point('products');
+        } else {
+            // Classic Grid.js version
+            include_once FORMIPAY_PATH . 'admin/page-products.php';
+        }
     }
 
     public function cpt_post_fields_box($boxes) {