feat(cluster): show the grid, and flag a new one

Finishes the half of this that was already computing on the backend and reaching
nobody. A Grid column in the Geo group, NEW GRID as a badge in Status, a filled
cell in the marker's own colour, and a filter chip.

new_grid joins lib/spotMarkers rather than getting colours of its own, so the
badge, the cell fill and the chip cannot drift apart - and the per-marker colour
setting will drive it with the rest from one table. Magenta: the last hue in the
categorical set not already spoken for, and one that does not read as a status,
because a new square is never urgent the way a new entity is.

The band map leaves it to the cluster, as it already leaves the prefix. The pill
is 22 px tall and its accent strip stops being readable past three segments.

A row carrying a new grid is no longer "dull", or the dimming would grey out the
one thing worth looking at.

The column is off by default, like the other Geo columns: it is only ever filled
for stations this receiver decoded over the UDP link, so for an operator who
does not run digital it would be a permanently empty column.

frontend/src/lib/spotDisplay.ts

@@ -31,6 +31,7 @@ type Entry = {
   new_county?: boolean;
   new_pota?: boolean;
   new_pfx?: boolean;
+  new_grid?: boolean;
 } | undefined;
 
 // applySpotDisplay rewrites a status entry per the options, so every consumer —

frontend/src/lib/spotMarkers.ts

@@ -15,7 +15,10 @@
 //                     statuses, blue is "callsign worked", green is POTA
 //   worked  blue    — matches the WKD-CALL badge the cluster list has always used
 //   prefix  yellow  — unchanged
-export type SpotMarkerKey = 'new_pota' | 'new_county' | 'new_pfx' | 'worked_call';
+//   grid    magenta — the last hue in the categorical set that is not already
+//                     spoken for here and does not read as a status; a grid is
+//                     never urgent the way a new entity is
+export type SpotMarkerKey = 'new_pota' | 'new_county' | 'new_pfx' | 'worked_call' | 'new_grid';
 
 export type SpotMarker = {
   key: SpotMarkerKey;
@@ -28,6 +31,7 @@ export const SPOT_MARKERS: SpotMarker[] = [
   { key: 'new_pota', colour: 'var(--success)', labelKey: 'clg2.newPota' },
   { key: 'new_county', colour: 'var(--chart-5)', labelKey: 'clg2.newCounty' },
   { key: 'new_pfx', colour: 'var(--caution)', labelKey: 'clg2.newPfx' },
+  { key: 'new_grid', colour: 'var(--chart-7)', labelKey: 'clg2.newGrid' },
   { key: 'worked_call', colour: 'var(--info)', labelKey: 'clg2.wkdCall' },
 ];