dashboard: pace a querying chart by its window, and an example to evaluate it

A chart is drawn in buckets, and nothing it can show changes until the
bucket it is drawing closes — so the resolution sets the refresh rather
than a flat five-second floor. A week at quarter-hour buckets now asks
four times an hour instead of sixty, for the same picture. Leaving the
field empty follows the window; a slower rate is still honoured.

`make seed-example` builds the thing to evaluate it with: a flow that
logs a temperature to InfluxDB, a flow that answers a chart's request by
turning the window into Flux and the rows back into a series, and a
dashboard holding the chart. The reading flow declares the request as an
input with a starting value, which is how a flow says a value reaches it
from a panel rather than from a node upstream.

Axis labels keep enough decimals to stay distinct — `si` rounds to three
figures, so every tick of a chart living inside one degree read "19".

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

frontend/src/components/Dashboard/panels.tsx

@@ -28,7 +28,7 @@ import {
   SelectValue,
 } from "@/components/ui/select"
 import { cn } from "@/lib/utils"
-import { MAX_SERIES, MIN_REFRESH_S } from "./ChartWidget"
+import { MAX_SERIES, refreshFor } from "./ChartWidget"
 import {
   CANVAS_PRESETS,
   COLUMN_CHOICES,
@@ -184,6 +184,13 @@ export function WidgetPanel({
   const series = seriesOf(widget)
   const setSeries = (next: Series[]) => set({ series: next })
   const querying = cfg.source === "query"
+  // What this chart would refresh at with nothing configured. The viewer can
+  // pick another window on the widget, which moves it.
+  const paced = refreshFor(
+    RANGES.find((range) => range.hours * 3600 === Number(cfg.range_s)) ??
+      DEFAULT_RANGE,
+    0,
+  )
 
   return (
     <SidePanel
@@ -364,15 +371,22 @@ export function WidgetPanel({
               <Label className="text-sm font-normal">Refresh, seconds</Label>
               <Input
                 type="number"
-                min={MIN_REFRESH_S}
-                value={str(cfg.refresh_s ?? 60)}
+                min={paced}
+                placeholder={str(paced)}
+                value={str(cfg.refresh_s ?? "")}
                 onChange={(event) =>
-                  set({ refresh_s: Number(event.target.value) || 0 })
+                  set({
+                    refresh_s:
+                      event.target.value === ""
+                        ? undefined
+                        : Number(event.target.value),
+                  })
                 }
               />
               <p className="text-xs text-muted-foreground">
-                How often it asks again. {MIN_REFRESH_S} seconds is the floor —
-                someone has to run the query.
+                Empty follows the window — {paced} seconds at this range, since
+                nothing changes until the bucket closes. A slower one is kept, a
+                faster one only asks for the same picture twice.
               </p>
             </div>
             <div className="grid gap-1.5">