Chart: an optional y axis title

The unit stays on the ticks; `yLabel` names the axis. It is part of the
chart's rebuild identity, so editing the title takes effect at once.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HTsT1isxUjw5gtkJk8WhuA

frontend/src/components/Dashboard/ChartWidget.tsx

@@ -67,6 +67,7 @@ function presentation(cfg: Record<string, unknown>) {
   const high = Number(cfg.y_max)
   return {
     unit: cfg.unit ? String(cfg.unit) : undefined,
+    yLabel: cfg.y_label ? String(cfg.y_label) : undefined,
     yRange:
       Number.isFinite(low) && Number.isFinite(high) && low < high
         ? ([low, high] as [number, number])