fix(cluster): group duplicates by call AND band AND mode

The grouping keyed on the callsign alone, so every spot of a station outside the
first one's band and mode was dropped from the list. RI1FJL spotted on 17m CW,
20m, 30m, 12m and 15m FT8 showed as a single row: four slots gone from the one
view an operator uses to find them.

A duplicate is the dozen skimmers that all heard the same CQ — same station,
same band, same mode. The same station on another band is the opposite of a
duplicate; it is what the chaser is scanning for.

frontend/src/App.tsx

@@ -5926,11 +5926,17 @@ export default function App() {
     });
     let rendered = list as (ClusterSpot & { repeats?: number })[];
     if (clusterGroup) {
+      // A DUPLICATE is the same station on the same band AND mode — the dozen
+      // skimmers that all heard one CQ. The same station on another band is the
+      // opposite of a duplicate: it is the line a DX chaser is scanning for, and
+      // grouping on the callsign alone deleted it. RI1FJL spotted on five bands
+      // showed as one row, so four slots simply vanished from the list.
       const seen = new Map<string, ClusterSpot & { repeats: number }>();
       for (const s of list) {
-        const e = seen.get(s.dx_call);
+        const key = `${(s.dx_call ?? '').toUpperCase()}|${(s.band ?? '').toLowerCase()}|${inferSpotMode(s.comment ?? '', s.freq_hz)}`;
+        const e = seen.get(key);
         if (e) { e.repeats++; }
-        else seen.set(s.dx_call, { ...s, repeats: 1 });
+        else seen.set(key, { ...s, repeats: 1 });
       }
       rendered = Array.from(seen.values());
     }