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).

frontend/src/components/SettingsModal.tsx

@@ -1246,6 +1246,7 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
   const [startEqEnd, setStartEqEnd] = useState(() => localStorage.getItem('opslog.startEqualsEnd') === '1');
   const [lookupOnBlur, setLookupOnBlur] = useState(() => localStorage.getItem('opslog.lookupOnBlur') === '1');
   const [groupDigital, setGroupDigital] = useState(() => localStorage.getItem('opslog.groupDigitalSlots') === '1');
+  const [clusterWorkedSameSlot, setClusterWorkedSameSlot] = useState(() => localStorage.getItem('opslog.clusterWorkedSameSlot') === '1');
   const [showQsoRate, setShowQsoRate] = useState(() => localStorage.getItem('opslog.showQsoRate') === '1');
   const [catModeBeforeFreq, setCatModeBeforeFreq] = useState(() => localStorage.getItem('opslog.catModeBeforeFreq') === '1');
   // Password-encryption (secret vault) state.
@@ -4000,6 +4001,11 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
             <span className="font-mono">dx.maritimecontestclub.net:7300</span>,{' '}
             <span className="font-mono">w8avi.net:7300</span>.
           </p>
+          <label className="flex items-start gap-2 text-sm cursor-pointer border-t border-border/60 pt-3">
+            <Checkbox checked={clusterWorkedSameSlot} className="mt-0.5"
+              onCheckedChange={(c) => { const v = !!c; setClusterWorkedSameSlot(v); writeUiPref('opslog.clusterWorkedSameSlot', v ? '1' : '0'); }} />
+            <span>{t('clu.workedSameSlot')} <span className="text-xs text-muted-foreground">{t('clu.workedSameSlotHint')}</span></span>
+          </label>
         </div>
 
         {editingServer && (