feat: add WPCFTO-inspired design system and React navigation

- Add WPCFTO-inspired design system CSS (colors, spacing, typography)
- Add reusable React components matching WPCFTO visual language
- Implement client-side navigation with hash-based routing
- Add NavigationMenu component for on-page navigation (no SSR)
- Update WordPress submenu highlighting based on current React page
- Add Refresh button to DataTable toolbar
- Fix icon imports in VariationPricingTable
- Remove page headers from all table pages (consistent UX)
- Convert Orders page to use DataTable for consistency

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

includes/Admin/ReactAdmin.php

@@ -65,6 +65,8 @@ class ReactAdmin {
             'ajaxUrl' => admin_url( 'admin-ajax.php' ),
             'restUrl' => rest_url( 'formipay/v1' ),
             'nonce' => wp_create_nonce( 'formipay-admin' ),
+            'pluginUrl' => FORMIPAY_URL,
+            'siteUrl' => site_url(),
         ] );
 
         // Debug logging
@@ -149,7 +151,7 @@ class ReactAdmin {
     public static function render_mount_point( $page ) {
 
         printf(
-            '<div id="formipay-admin-root" data-formipay-mount="%s">Loading %s...</div>',
+            '<div class="wrap"><div id="formipay-admin-root" data-formipay-mount="%s">Loading %s...</div></div>',
             esc_attr( $page ),
             esc_html( ucfirst( $page ) )
         );