Draw the panel as a panel

Five things a wall panel showed that a wall panel should not.

**A tile's body no longer clips.** It scrolled, and a box that scrolls
also cuts whatever crosses its edge — which took the glow off a lit
button at exactly the width where the button filled its tile, and off a
gauge's arc at exactly the height where the dial filled its own. Only
what is written or listed asks for a scroller now; everything else is a
picture drawn to fit, and what overflows is left to the frame, which
clips at the tile's edge where a shadow has already faded out. The
slider's phantom scrollbar goes with it.

**The selector is a selector.** Named for what it does rather than what
it is, and the choice it is holding is held in the dashboard's own
primary — a pill that slides between the options rather than a grey one
that had to be looked for. The stored type is untouched, so no document
changes meaning.

**The arrangement is held off the panel's edges**, by the same distance
it holds between two widgets. The ground is not held off with it: a
background covers the whole panel, and only what is arranged on it has
a margin. No stored panel loses a row to it.

**The rail is drawn on the panel.** It was chrome bolted to the edge of
the screen beside the canvas — in the app's own design rather than the
dashboard's, and on a scaled canvas not even lined up with it. It now
takes a column out of the canvas the way the margin does, scaled with
it and wearing its look. Which cell each widget sits in is unchanged;
only how big a cell is.

Two of these were the same mistake twice: an unlayered rule stating
`position` for everything wearing a class — `.gl-surface` on a rail
placed by a utility, and a blanket lift over every child of a pressable
on a pill placed by `layoutId`. Both now say it one element at a time.

docs/interface/dashboards.md

@@ -49,7 +49,7 @@ screen reader calls its controls, and what a published value is labelled with.
 | **Switch** | `bool` | on/off |
 | **Slider** | `float`, `int` | min, max, step |
 | **Input** | text or a number | free entry |
-| **Dropdown** | one of a list | a mode, a scene, a preset |
+| **Selector** | one of a list | a mode, a scene, a preset — as a menu, or as a row of choices with the active one held |
 | **Colour** | `[h, s, v]`, `[r, g, b]` or `"#rrggbb"` | a hue wheel with saturation and brightness, for an RGB fixture |
 
 The colour wheel sends `[h, s, v]` by default — hue 0-360 degrees, saturation