Draw the error series terracotta on Home

Failures were slot 1 of a ramp that carries identity by lightness alone, so at
a 2px stroke the line that matters read as one picture with avg and lag. It
takes --brand-secondary now — the same terracotta the brain graph above it
already spends on a flow that cannot run. A palette entry naming a token is
resolved off the chart's element, since a canvas cannot parse var().

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/components/Common/UplotChart.tsx

@@ -176,13 +176,16 @@ function token(name: string, from: Element): string {
 /**
  * What one line is drawn in.
  *
- * A palette entry is either a slot of the app's own ramp — the shape a
- * dashboard stored before it could name colours — or a colour, which is what a
- * palette writes today. A colour is used as it stands.
+ * A palette entry is one of three things: a slot of the app's own ramp — the
+ * shape a dashboard stored before it could name colours — a token, for a line
+ * that means something the ramp has no step for, or a colour, which is what a
+ * dashboard palette writes today. A colour is used as it stands; the other two
+ * are resolved here, since the canvas cannot read a custom property.
  */
 const seriesColor = (index: number, slots: string[], from: Element) => {
   const slot = slots[index % slots.length]
-  return CHART_SLOTS.includes(slot) ? token(`--chart-${slot}`, from) : slot
+  if (CHART_SLOTS.includes(slot)) return token(`--chart-${slot}`, from)
+  return slot.startsWith("--") ? token(slot, from) : slot
 }
 
 /**