feat: add subject validation for message types and display error if missing

ui/ui/admin/customers/list.tpl

@@ -274,6 +274,16 @@
                 return;
             }
 
+            if (messageType == 'all' || messageType == 'inbox' || messageType == 'email' && !subject) {
+                Swal.fire({
+                    title: 'Error!',
+                    text: "{Lang::T('Please enter a subject for the message.')}",
+                    icon: 'error',
+                    confirmButtonText: 'OK'
+                });
+                return;
+            }
+
             // Disable the button and show loading text
             $(this).prop('disabled', true).text('{Lang::T('Sending...')}');