feat(cluster): "already worked only on the same slot" option

New DX Cluster setting (Settings → DX Cluster). Off (today's behaviour) a call
worked anywhere reads as already worked; on, the WORKED-call flag needs the same
band AND mode. It folds through the digital-mode grouping (Settings → General):
grouped, a 20m FT8 contact also marks a 20m FT4 spot worked; ungrouped they are
separate slots. Backend: qso.WorkedCallSlotKeys builds CALL|band and
CALL|band|mode(grouped) keys; ClusterSpotStatuses uses them when the option is on
(a spot with no inferable mode falls back to same-band).

app.go

@@ -2400,6 +2400,19 @@ func (a *App) groupDigitalSlots() bool {
 	return v == "1"
 }
 
+// clusterWorkedSameSlot reports the "consider a call already worked only if
+// worked on the SAME band+mode slot" cluster preference (Settings → DX Cluster).
+// Off (default) → a call worked on any band/mode reads as already worked. On →
+// the WORKED-call flag needs the same band and mode (digital-grouped when that
+// option is also on).
+func (a *App) clusterWorkedSameSlot() bool {
+	if a.settings == nil {
+		return false
+	}
+	v, _ := a.settings.Get(a.ctx, "ui.opslog.clusterWorkedSameSlot")
+	return v == "1"
+}
+
 func (a *App) GetUIPref(key string) (string, error) {
 	if a.settings == nil || !a.settingsScoped.Load() {
 		// Distinct from a genuinely-empty pref: the (LOCAL SQLite) settings store
@@ -15925,6 +15938,14 @@ func (a *App) ClusterSpotStatuses(spots []SpotQuery) []SpotStatus {
 	// "I've already QSO'd this exact station" even when the band/mode
 	// makes the entity check say "new-band" or "new-slot".
 	workedCalls, _ := a.qso.WorkedCallsigns(a.ctx)
+	// "Already worked only on the same slot" option (Settings → DX Cluster): the
+	// WORKED-call flag then needs the SAME band and mode (digital-grouped through
+	// the same normMode when that option is on) rather than the call anywhere.
+	sameSlot := a.clusterWorkedSameSlot()
+	var workedCallSlots map[string]struct{}
+	if sameSlot {
+		workedCallSlots, _ = a.qso.WorkedCallSlotKeys(a.ctx, normMode)
+	}
 	// Orthogonal dimensions: worked US counties (for the ULS callsign→county
 	// lookup) and worked POTA parks. Both built once per batch.
 	workedCounties, _ := a.qso.WorkedCountyKeys(a.ctx, award.USCountyKey)
@@ -15945,7 +15966,21 @@ func (a *App) ClusterSpotStatuses(spots []SpotQuery) []SpotStatus {
 			Band: strings.ToLower(q.Band),
 			Mode: strings.ToUpper(q.Mode),
 		}
-		if _, ok := workedCalls[strings.ToUpper(q.Call)]; ok {
+		if sameSlot {
+			// Already worked ONLY when this exact band+mode slot was worked. With no
+			// inferable mode, fall back to same-band (better than claiming the whole
+			// call is fresh). Digital grouping folds through the same normMode.
+			upCall := strings.ToUpper(q.Call)
+			if out[i].Mode == "" {
+				_, out[i].WorkedCall = workedCallSlots[upCall+"|"+out[i].Band]
+			} else {
+				cm := out[i].Mode
+				if normMode != nil {
+					cm = normMode(cm)
+				}
+				_, out[i].WorkedCall = workedCallSlots[upCall+"|"+out[i].Band+"|"+cm]
+			}
+		} else if _, ok := workedCalls[strings.ToUpper(q.Call)]; ok {
 			out[i].WorkedCall = true
 		}
 		// NEW PFX: the spot's CQ WPX prefix, never worked before.