feat(cluster): configurable spot lifetime

Spots were bounded by COUNT alone, so on a quiet band a two-hour-old spot sat
on the band map looking like something to chase. Settings → DX Cluster now
takes a lifetime: presets at 5/10/15/30/60 minutes, or any value up to twelve
hours, and 0 keeps the old behaviour.

Spots are REMOVED from the shared list rather than hidden at render. The
cluster list, every band map and the counts all read that one array, so pruning
it once is what makes the setting mean the same thing in every view — the
lesson already paid for when the band map ignored the cluster's filters. It
also gives the memory back.

A spot whose timestamp will not parse is never dropped: an unreadable stamp is
a reason to distrust the clock, not to throw away the spot.

Swept every 30 seconds — close enough that a 5-minute setting is honoured, and
invisible next to the spot stream. Clamped in the backend as well as the UI.

frontend/wailsjs/go/main/App.d.ts

@@ -506,6 +506,8 @@ export function GetSlotStats():Promise<qso.SlotStats>;
 
 export function GetSolarData():Promise<solar.Data>;
 
+export function GetSpotTTLMinutes():Promise<number>;
+
 export function GetStartupStatus():Promise<main.StartupStatus>;
 
 export function GetStationDevices():Promise<Array<main.StationDevice>>;
@@ -1000,6 +1002,8 @@ export function SetPassphrase(arg1:string):Promise<void>;
 
 export function SetScpEnabled(arg1:boolean):Promise<void>;
 
+export function SetSpotTTLMinutes(arg1:number):Promise<void>;
+
 export function SetTelemetryEnabled(arg1:boolean):Promise<void>;
 
 export function SetUIPref(arg1:string,arg2:string):Promise<void>;