fix(dashboard): fix temperature widget undersampling on long ranges over sparse data

Summary

• Size the temperature widget's InfluxDB aggregation window off the actual data span instead of the full selected range, so a long range picked over young/sparse data no longer renders as just a handful of coarse points.
• Show a one-time toast (Panel's built-in notification system) naming the oldest data point found, when the selected range reaches further back than any recorded data.