fix: improve SearchableSelect with label resolution for pre-selected items

- Fetch post titles for pre-selected IDs on mount via include[] param
- Cache labels from search results to avoid re-fetching
- Filter already-selected items from dropdown results
- Add loading spinner and no-results message
- Update PHP autocomplete handler to support include[] param
- Fix CSS class conflict warning

includes/Coupon.php

@@ -1073,8 +1073,35 @@ class Coupon {
 
         $post_type = isset($_REQUEST['post_type']) ? sanitize_text_field(wp_unslash($_REQUEST['post_type'])) : '';
         $search = isset($_REQUEST['search']) ? sanitize_text_field(wp_unslash($_REQUEST['search'])) : '';
+        $include = isset($_REQUEST['include']) ? array_map('intval', (array) $_REQUEST['include']) : [];
 
-        if (empty($post_type) || strlen($search) < 2) {
+        if (empty($post_type)) {
+            wp_send_json_error( [ 'message' => 'Invalid request' ] );
+        }
+
+        // Resolve labels for specific IDs (pre-selected items)
+        if (!empty($include)) {
+            $query = get_posts([
+                'post_type' => $post_type,
+                'post__in' => $include,
+                'posts_per_page' => -1,
+                'post_status' => 'any',
+            ]);
+            $results = [];
+            if (!empty($query)) {
+                foreach ($query as $post) {
+                    $results[] = [
+                        'value' => $post->ID,
+                        'label' => $post->post_title,
+                    ];
+                }
+            }
+            wp_send_json_success($results);
+            return;
+        }
+
+        // Search by keyword
+        if (strlen($search) < 2) {
             wp_send_json_error( [ 'message' => 'Invalid request' ] );
         }