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/wailsjs/go/main/App.js

@@ -1066,6 +1066,10 @@ export function GetSpotColors() {
   return window['go']['main']['App']['GetSpotColors']();
 }
 
+export function GetSpotMax() {
+  return window['go']['main']['App']['GetSpotMax']();
+}
+
 export function GetSpotTTLMinutes() {
   return window['go']['main']['App']['GetSpotTTLMinutes']();
 }
@@ -2238,6 +2242,10 @@ export function SetScpEnabled(arg1) {
   return window['go']['main']['App']['SetScpEnabled'](arg1);
 }
 
+export function SetSpotMax(arg1) {
+  return window['go']['main']['App']['SetSpotMax'](arg1);
+}
+
 export function SetSpotTTLMinutes(arg1) {
   return window['go']['main']['App']['SetSpotTTLMinutes'](arg1);
 }