Enlarging the activity heatmap in the previous commit inverted the section's
hierarchy: the heatmap became the heaviest block on the tab even though it is
the secondary read, a weekday pattern, while the bar chart carries magnitude
over time and was only gridUnit * 3 tall with up to 90 bars in it.

InteractiveChart gains a plotHeight property defaulting to its current size,
so the inline provider detail charts are unchanged. The Usage & Spend chart
opts into gridUnit * 6, and the heatmap cell cap drops to gridUnit * 1.15.
Individual daily spikes are now legible and the chart outranks the heatmap.

Verified with plasmawindowed against live CLI data.