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

scripts/seed_demo.py

@@ -901,6 +901,9 @@ ENERGY_WIDGETS = [
             "history": {"points": 720},
             "unit": " kW",
             "y_label": "kW",
+            # Twelve hours of readings a minute apart: the shape is what is
+            # being read, not any single sample.
+            "smooth": True,
             # Fixed, so the picture does not rescale under you every time the
             # sun goes behind something. Negative is exporting.
             "y_min": -7,
@@ -914,8 +917,8 @@ ENERGY_WIDGETS = [
         "layout": at(7, 0, 6, 4),
         "config": {
             # The other kind of chart: it asks, rather than reading the ring
-            # the engine keeps. The range picker at the top of the tile is what
-            # publishes the request.
+            # the engine keeps. The range picker on the tile's title line is
+            # what publishes the request.
             "source": "query",
             "request": msg(HISTORY, "chart_request"),
             "request_dtype": "record",