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>

includes/Admin/ReactAdmin.php

@@ -26,6 +26,12 @@ class ReactAdmin {
             return;
         }
 
+        // Check coexistence mode - only load React when ?react=1 or option is set
+        $use_react = isset($_GET['react']) || get_option('formipay_use_react_admin', false);
+        if ( ! $use_react ) {
+            return; // Classic mode - don't load React assets
+        }
+
         // Enqueue React build assets
         $build_dir = FORMIPAY_PATH . 'build';
         $build_url = FORMIPAY_URL . 'build';