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/themes/base.css

@@ -9,13 +9,8 @@
   --series-6: #008300;
   --series-7: #9085e9;
   --series-8: #e66767;
-  --seq-0: color-mix(in srgb, var(--fg1) 9%, var(--bg1));
-  --seq-1: #104281;
-  --seq-2: #184f95;
-  --seq-3: #256abf;
-  --seq-4: #3987e5;
-  --seq-5: #6da7ec;
-  --seq-6: #9ec5f4;
+  --heat-empty: color-mix(in srgb, var(--fg1) 9%, var(--bg1));
+  --heat-full: var(--accent);
 }
 :root[data-mode="light"] {
   --series-1: #2a78d6;
@@ -26,13 +21,8 @@
   --series-6: #008300;
   --series-7: #6250d6;
   --series-8: #e34948;
-  --seq-0: color-mix(in srgb, var(--fg1) 9%, var(--bg1));
-  --seq-1: #cde2fb;
-  --seq-2: #9ec5f4;
-  --seq-3: #6da7ec;
-  --seq-4: #3987e5;
-  --seq-5: #256abf;
-  --seq-6: #184f95;
+  --heat-empty: color-mix(in srgb, var(--fg1) 9%, var(--bg1));
+  --heat-full: var(--accent);
 }
 
 /* Theme-independent tokens. */