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/App.tsx

@@ -1487,7 +1487,7 @@ export default function App() {
   // is an OR across all of them, as it already was for a worked callsign.
   // 'new-call' is a DISPLAY status, produced by the slot-highlight option rather
   // than by the backend, so it is named here and not in SpotStatusKey.
-  type SpotFilterKey = SpotStatusKey | 'new-pota' | 'new-county' | 'new-pfx' | 'new-call';
+  type SpotFilterKey = SpotStatusKey | 'new-pota' | 'new-county' | 'new-pfx' | 'new-call' | 'new-grid';
   // Display options, shared with the band map through lib/spotDisplay. Kept in
   // localStorage (mirrored to settings by writeUiPref) so both panels and the
   // filter predicate read one source without prop-drilling through three levels.
@@ -4713,7 +4713,8 @@ export default function App() {
           || (!!e?.worked_call && clusterStatusFilter.has('worked'))
           || (!!e?.new_pota && clusterStatusFilter.has('new-pota'))
           || (!!e?.new_county && clusterStatusFilter.has('new-county'))
-          || (!!e?.new_pfx && clusterStatusFilter.has('new-pfx'));
+          || (!!e?.new_pfx && clusterStatusFilter.has('new-pfx'))
+          || (!!e?.new_grid && clusterStatusFilter.has('new-grid'));
         if (!matches) return false;
       }
       // LoTW only, and the spotter's continent. Both are properties of the
@@ -4893,6 +4894,8 @@ export default function App() {
               { k: 'new-pota'   as SpotFilterKey, label: 'NEW POTA',   cls: 'bg-success-muted text-success-muted-foreground border-success-border' },
               { k: 'new-county' as SpotFilterKey, label: 'NEW COUNTY', cls: 'bg-success-muted text-success-muted-foreground border-success-border' },
               { k: 'new-pfx'    as SpotFilterKey, label: 'NEW PFX',    cls: 'bg-caution-muted text-caution-muted-foreground border-caution-border' },
+              // Only ever set for a station this receiver decoded over the UDP link.
+              { k: 'new-grid'   as SpotFilterKey, label: 'NEW GRID',   cls: 'bg-muted text-foreground border-border' },
               // Blue, like the already-worked call in the grid. Selecting it keeps
               // worked spots; the separate "Hide worked" checkbox drops them — they
               // are opposite controls, so don't use both at once.