Stats: heatmap uses the theme accent with a continuous color ramp

Cells blend in OKLab from a neutral empty color to the theme's accent on a
square-root scale instead of seven fixed blue steps; the legend shows the
gradient.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/styles.css

@@ -1066,6 +1066,9 @@ table.grid input[type="number"] {
   border-radius: 3px;
   display: inline-block;
 }
+.legend i.heat-scale {
+  width: 120px;
+}
 .tooltip {
   position: absolute;
   pointer-events: none;