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 && (

frontend/src/lib/i18n.tsx

@@ -262,6 +262,8 @@ const en: Dict = {
   'clu.moveUp': 'Move up', 'clu.moveDown': 'Move down', 'clu.edit': 'Edit', 'clu.delete': 'Delete', 'clu.none': 'No cluster nodes saved yet.', 'clu.connect': 'Connect', 'clu.disconnect': 'Disconnect',
   'clu.add': 'Add cluster', 'clu.connectAll': 'Connect all', 'clu.disconnectAll': 'Disconnect all', 'clu.autoConnect': 'Auto-connect all enabled on app start',
   'clu.freeNodes': 'Free public nodes:',
+  'clu.workedSameSlot': 'Already worked only on the same slot',
+  'clu.workedSameSlotHint': '— a spot shows "worked" only if you worked that call on the SAME band and mode, not just anywhere. Combines with digital-mode grouping (Settings → General): with it on, a call worked on 20m FT8 also counts as worked for a 20m FT4 spot; with it off, FT8 and FT4 are separate slots.',
   // Backup panel
   'bk.hintMysql': 'On close (once/day) OpsLog snapshots the local SQLite (config) AND exports the shared MySQL log to ADIF — opslog-log-<date>.adi — so your contacts are protected even though they live on the server. Rotation keeps the last N of each.',
   'bk.hint': 'OpsLog can copy the SQLite database to a folder of your choice when you close it, once per day. Rotation keeps the last N copies and deletes older ones.',
@@ -672,6 +674,8 @@ const fr: Dict = {
   'clu.moveUp': 'Monter', 'clu.moveDown': 'Descendre', 'clu.edit': 'Éditer', 'clu.delete': 'Supprimer', 'clu.none': 'Aucun nœud cluster enregistré.', 'clu.connect': 'Connecter', 'clu.disconnect': 'Déconnecter',
   'clu.add': 'Ajouter cluster', 'clu.connectAll': 'Tout connecter', 'clu.disconnectAll': 'Tout déconnecter', 'clu.autoConnect': 'Connexion auto de tous les activés au démarrage',
   'clu.freeNodes': 'Nœuds publics gratuits :',
+  'clu.workedSameSlot': 'Déjà contacté seulement sur le même slot',
+  'clu.workedSameSlotHint': '— un spot n\'affiche « contacté » que si vous avez contacté cet indicatif sur la MÊME bande et le MÊME mode, pas juste n\'importe où. Se combine avec le groupage des modes numériques (Réglages → Général) : activé, un indicatif contacté en 20m FT8 compte aussi comme contacté pour un spot 20m FT4 ; désactivé, FT8 et FT4 sont des slots distincts.',
   'bk.hintMysql': "À la fermeture (1×/jour) OpsLog sauvegarde le SQLite local (config) ET exporte le log MySQL partagé en ADIF — opslog-log-<date>.adi — pour protéger tes contacts même s'ils sont sur le serveur. La rotation garde les N derniers de chaque.",
   'bk.hint': "OpsLog peut copier la base SQLite dans un dossier de ton choix à la fermeture, une fois par jour. La rotation garde les N dernières copies et supprime les plus anciennes.",
   'bk.auto': 'Sauvegarde auto à la fermeture d\'OpsLog (max 1×/jour)', 'bk.folder': 'Dossier de sauvegarde', 'bk.folderPh': 'vide = dossier par défaut', 'bk.browse': 'Parcourir…',