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.
