Backend: wrap SUM() queries with COALESCE in handle_time_series, handle_clients_usage, and handle_detailed_usage to prevent NULL-induced panics when no data exists for a time window. Frontend: add showEmptyChart() empty-state messages and error feedback across overview, analytics, costs, and clients pages. Rewrite analytics loadCharts() to use Promise.allSettled() so each chart renders independently on partial API failures.