Chart cursor, range picker in the header, line smoothing

A dashboard canvas is CSS-scaled to fit its panel while uPlot maps the
pointer against its own unscaled plot width, so the cursor drifted further
right the further into a chart it went. A `cursor.move` refiner divides the
visual offset back into layout pixels; unscaled hosts get a no-op.

A querying chart's range picker moves onto the frame's title line through a
new `useHeaderSlot`, giving the plot back the row it spent. The editor's drag
handle is the header, so the picker is exempted from it.

Charts can be drawn as a monotone cubic spline — uPlot's own path builder,
monotone so a smoothed line never invents a reading between two real ones.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016ZeGnqVsf5VHQqvz4HdUhN

frontend/src/components/Dashboard/panels.tsx

@@ -647,6 +647,24 @@ export function WidgetPanel({
           </div>
         ) : null}
 
+        {widget.type === "chart" ? (
+          <div className="grid gap-1.5">
+            <div className="flex items-center justify-between gap-2 text-sm">
+              Smoothing
+              <Switch
+                checked={Boolean(cfg.smooth)}
+                aria-label="Smoothing"
+                data-testid="chart-smooth"
+                onCheckedChange={(smooth) => set({ smooth })}
+              />
+            </div>
+            <p className="text-xs text-muted-foreground">
+              Curves the lines between readings. It draws them softer; it does
+              not change what was measured.
+            </p>
+          </div>
+        ) : null}
+
         {widget.type === "stat" ||
         widget.type === "gauge" ||
         widget.type === "chart" ||