Spread a chart's default colours across the ramp

A chart that named no palette took the ramp in order, so three lines drew
slots 1, 2 and 3 — adjacent steps of a ramp that carries identity by lightness
alone, which at a 2px stroke read as close to one picture. Each count now
takes the slots furthest apart that the ramp allows: 1 -> 1, 2 -> 1/5,
3 -> 1/3/5, 4 -> 1/2/4/5. Five lines are unchanged.

Moving a default is only safe because the mechanism around it is inert: a
palette a dashboard wrote down is still drawn exactly as written, so no stored
document means anything different than it did. `palette.check.ts` now pins the
spread table and says which of the two properties is permanent.

An unnamed palette is empty rather than the whole ramp, so the settings panel
shows nothing picked when a dashboard is on automatic — and deselecting the
last colour is now the way back to it.

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

@@ -1242,10 +1242,6 @@ export function DashboardPanel({
                     type="button"
                     aria-pressed={picked}
                     aria-label={`Colour ${slot}`}
-                    // The last one cannot go. An empty palette falls back to
-                    // the whole ramp, so the row would then contradict what
-                    // the charts beside it are drawing.
-                    disabled={picked && palette.length === 1}
                     onClick={() =>
                       setSetting("palette", {
                         ...paletteSetting,
@@ -1273,9 +1269,10 @@ export function DashboardPanel({
             <p className="text-sm text-muted-foreground">
               The colours this dashboard's charts draw with, taken in the order
               you pick them — a chart with more lines than colours starts over
-              at the first. Each is offered once: two lines sharing a colour
-              could not be told apart, and neighbouring ones are close enough
-              already.
+              at the first. Pick none and each chart spreads itself across the
+              range by how many lines it has, which is usually what you want.
+              Each colour is offered once: two lines sharing one could not be
+              told apart, and neighbouring ones are close enough already.
             </p>
           </div>