refactor: remove coexistence mode, use React only

Remove all dual-mode rendering logic since React is now the single
admin interface. Focus on implementing full table features in React.

Changes:
- Remove ?react= query param checks from all page methods
- Remove admin notice and footer toggle from ReactAdmin
- Simplify asset loading - ReactAdmin handles all assets
- Clean up Init.php enqueue method

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

includes/Coupon.php

@@ -94,26 +94,13 @@ class Coupon {
     }
 
     public function formipay_coupon() {
-        // 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('coupons');
-        } else {
-            // Classic Grid.js version
-            include_once FORMIPAY_PATH . 'admin/page-coupons.php';
-        }
+        // React admin
+        \Formipay\Admin\ReactAdmin::render_mount_point('coupons');
     }
 
     public function enqueue_admin() {
-        global $current_screen;
-
-        // Check coexistence mode - if React is active, don't load classic assets
-        $use_react = isset($_GET['react']) || get_option('formipay_use_react_admin', false);
-        if ($use_react) {
-            return;
-        }
+        // Assets now handled by ReactAdmin class
+        return;
         
         if($current_screen->id == 'formipay_page_formipay-coupons') {