feat(cluster): the spot list size is a setting

The list is a ring buffer that held a thousand spots, and on a busy
evening a thousand arrive in a couple of minutes. So the buffer decided
how long a spot lived, and the spot LIFETIME setting never got the chance
to expire anything: fifteen minutes meant nothing when the oldest spot
was pushed out after two.

Now settable (Preferences → Cluster, beside the lifetime, since between
them they decide the same thing), 100 to 10 000. The ceiling is a real
limit rather than a round number: every spot is matched against the
worked index and the alert rules, and is a row the cluster grid and every
open band map re-render.

frontend/src/App.tsx

@@ -95,7 +95,7 @@ import { ShutdownProgress } from '@/components/ShutdownProgress';
 import { ClusterGrid } from '@/components/ClusterGrid';
 import { cleanSpotter, inferSpotMode, spotModeCategory, spotStatusKey } from '@/lib/spot';
 import { applySpotDisplay, readSpotDisplayOptions, spotIsWorked, SPOT_DISPLAY_OPTIONS_EXPOSED } from '@/lib/spotDisplay';
-import { AnswerDecode, HaltDecodeTx, LogUIError, FlexTXOnBand, GetMatrixColors, GetRotorPresets, GetRowColors, GetSpotTTLMinutes, IsNewUSCounty } from '../wailsjs/go/main/App';
+import { AnswerDecode, HaltDecodeTx, LogUIError, FlexTXOnBand, GetMatrixColors, GetRotorPresets, GetRowColors, GetSpotTTLMinutes, GetSpotMax, IsNewUSCounty } from '../wailsjs/go/main/App';
 import { applyMatrixColors } from '@/lib/matrixColors';
 import { WorkedBeforeGrid } from '@/components/WorkedBeforeGrid';
 import { NetControlPanel } from '@/components/NetControlPanel';
@@ -1663,7 +1663,14 @@ export default function App() {
   const [clusterServers, setClusterServers] = useState<{ id: number; name: string; enabled: boolean; sort_order: number }[]>([]);
   // Ring buffer — only keep the last N spots; cluster firehose can be heavy.
   const [spots, setSpots] = useState<ClusterSpot[]>([]);
-  const SPOTS_CAP = 1000;
+  // How many spots the list holds. A setting rather than a constant: at a
+  // thousand, a busy evening filled the buffer in a couple of minutes, so the
+  // spot LIFETIME never had anything left to expire — the cap was deciding the
+  // lifetime. Kept in a ref as well: the append path runs inside a state updater
+  // that must not close over a stale value.
+  const [spotsCap, setSpotsCap] = useState(1000);
+  const spotsCapRef = useRef(1000);
+  useEffect(() => { spotsCapRef.current = spotsCap; }, [spotsCap]);
   // Cluster filter selections persist across restarts (writeUiPref → localStorage
   // + DB, so they also travel with a copied data/ folder). Loaders read the cache
   // synchronously at first render; a single effect below writes them back.
@@ -1977,7 +1984,7 @@ export default function App() {
   useEffect(() => {
     setSpotStatus((prev) => {
       const keys = Object.keys(prev);
-      if (keys.length <= SPOTS_CAP * 2) return prev;
+      if (keys.length <= spotsCapRef.current * 2) return prev;
       const live = new Set(spots.map((x) => spotStatusKey(x.dx_call, x.band ?? '', x.comment ?? '', x.freq_hz)));
       // Decoded stations count as live too. They share this cache, and pruning
       // to the cluster spots alone would evict every one of them — on a busy
@@ -2488,6 +2495,9 @@ export default function App() {
   // same thing everywhere — and it gives the memory back.
   const [spotTTLMin, setSpotTTLMin] = useState(0);
   useEffect(() => { GetSpotTTLMinutes().then(setSpotTTLMin).catch(() => {}); }, [showSettings]);
+  // Same beat as the lifetime: re-read when Preferences closes, since the two
+  // settings decide between them how long a spot survives.
+  useEffect(() => { GetSpotMax().then((n: number) => { if (n > 0) setSpotsCap(n); }).catch(() => {}); }, [showSettings]);
   useEffect(() => {
     if (spotTTLMin <= 0) return; // 0 = keep until the count cap pushes them out
     const sweep = () => {
@@ -3478,7 +3488,8 @@ export default function App() {
           const filtered = hist(sp) ? next : next.filter((x) => hist(x) || key(x) !== k);
           next = [sp, ...filtered];
         }
-        return next.length > SPOTS_CAP ? next.slice(0, SPOTS_CAP) : next;
+        const cap = spotsCapRef.current;
+        return next.length > cap ? next.slice(0, cap) : next;
       });
     };
     const unsubSpot = EventsOn('cluster:spot', (sp: ClusterSpot) => {