fix(master-dashboard): Resolve trainers table AJAX loading error

- Initialize HVAC_Master_Dashboard_Data during plugin init to ensure
  AJAX handler is registered for all requests (not just template loads)
- Accept both hvac_ajax_nonce and hvac_master_dashboard_nonce for
  backward compatibility with existing JavaScript

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

includes/class-hvac-master-dashboard-data.php

@@ -850,9 +850,16 @@ class HVAC_Master_Dashboard_Data {
 	 * AJAX handler for trainers table
 	 */
 	public function ajax_get_trainers_table() {
-		// Check nonce
-		if ( ! isset( $_POST['nonce'] ) || ! wp_verify_nonce( $_POST['nonce'], 'hvac_master_dashboard_nonce' ) ) {
-			wp_die( 'Security check failed' );
+		// Check nonce - accept both hvac_ajax_nonce and hvac_master_dashboard_nonce for compatibility
+		if ( ! isset( $_POST['nonce'] ) ) {
+			wp_die( 'Security check failed: No nonce provided' );
+		}
+
+		$nonce_valid = wp_verify_nonce( $_POST['nonce'], 'hvac_ajax_nonce' ) ||
+		               wp_verify_nonce( $_POST['nonce'], 'hvac_master_dashboard_nonce' );
+
+		if ( ! $nonce_valid ) {
+			wp_die( 'Security check failed: Invalid nonce' );
 		}
 		
 		// Check permissions

includes/class-hvac-plugin.php

@@ -500,7 +500,12 @@ final class HVAC_Plugin {
         if (class_exists('HVAC_Access_Control')) {
             new HVAC_Access_Control();
         }
-        
+
+        // Initialize Master Dashboard Data to register AJAX handlers
+        if (class_exists('HVAC_Master_Dashboard_Data')) {
+            new HVAC_Master_Dashboard_Data();
+        }
+
         // Initialize optional components
         $this->initializeOptionalComponents();