Cost/metrics widgets: auto-adjust time segments for visual clarity

Cost dashboard histograms should auto-adjust bucket width based on the selected time range. 1 hour → 5-min buckets, 24 hours → 1-hour buckets, 7 days → 6-hour buckets, 30 days → 1-day buckets. The visualization should look good at any zoom level without manual configuration. Applies to ai/cost time-series data and any metrics widgets.