perf(cluster): widen the spot batching window with the rate of the feed

Every flush commits state the whole window re-renders on, and the window was a
flat 50 ms — twenty full renders a second under an RBN firehose. That is paid
everywhere else in the interface, and it is what a dropdown highlighting its
entries a beat behind the mouse actually is.

The window now follows the rate: 50 ms on a quiet cluster, where a spot should
appear the moment it arrives; 200 ms past five a second; 500 ms past twenty,
where half a second's delay on a line already scrolling past is invisible.

frontend/src/App.tsx

@@ -2068,6 +2068,9 @@ export default function App() {
   useEffect(() => { spotStatusRef.current = spotStatus; }, [spotStatus]);
   // Mirror of spots so the log-triggered refresh reads the current list without
   // a stale closure.
+  // Arrival times of the last second's spots, for the adaptive batching window
+  // in the cluster:spot listener.
+  const spotRateRef = useRef<number[]>([]);
   const spotsRef = useRef(spots);
   useEffect(() => { spotsRef.current = spots; }, [spots]);
   // The decoded stations, for the same reason: the status refresh and the cache
@@ -3595,11 +3598,24 @@ export default function App() {
     const unsubSpot = EventsOn('cluster:spot', (sp: ClusterSpot) => {
       // Stage the spot; a short timer resolves its status then commits it.
       pendingSpotsRef.current.push(sp);
-      // 50 ms is enough to coalesce an RBN burst into one status lookup (the
-      // worked-index is in memory, so resolving is near-instant) while staying
-      // imperceptible.
+      // The window WIDENS with the rate of the feed.
+      //
+      // Every flush commits state that the whole window re-renders on, so a
+      // fixed 50 ms means twenty full renders a second under an RBN firehose —
+      // and that is felt everywhere else: a dropdown highlighting its entries a
+      // beat late as the mouse moves down them, which is what was reported.
+      //
+      // A quiet cluster keeps the 50 ms: a handful of spots an hour should
+      // appear the moment they arrive. A busy one is coalesced instead, and half
+      // a second's delay on a line in a list that is already scrolling past is
+      // not something anyone can see.
+      const now = Date.now();
+      spotRateRef.current = spotRateRef.current.filter((t) => now - t < 1000);
+      spotRateRef.current.push(now);
+      const perSec = spotRateRef.current.length;
+      const window_ms = perSec > 20 ? 500 : perSec > 5 ? 200 : 50;
       if (pendingSpotTimer.current === undefined) {
-        pendingSpotTimer.current = window.setTimeout(flushPendingSpots, 50);
+        pendingSpotTimer.current = window.setTimeout(flushPendingSpots, window_ms);
       }
       // Self-spot: someone spotted OUR callsign — show it in the shared header
       // toast (same place as the other notifications), not a separate banner.