feat(cluster): add a WORKED status-filter chip

The filter logic already matched worked_call spots against a 'worked' key; add
the chip to the status row so worked spots can be shown or isolated, not only
dropped via the "Hide worked" checkbox.

frontend/src/App.tsx

@@ -4619,7 +4619,10 @@ 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' },
-              // (no WORKED chip — use the "Hide worked" checkbox to drop dupes.)
+              // 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.
+              { k: 'worked'   as SpotFilterKey, label: 'WORKED',   cls: 'bg-info-muted text-info-muted-foreground border-info-border' },
             ]).map((s) => {
               const on = clusterStatusFilter.has(s.k);
               return (