[DX Assistant] Wire DXClusterAssistant into MainWindow #1013

std::unique_ptr<DXClusterAssistant>  dxClusterAssistant;
DXAssistantEngine                   *dxAssistantEngine   = nullptr;
ClubLogMostWanted                   *clubLogMostWanted   = nullptr;
QString                              myContinent;         // derived once at startup
QAction                             *dxClusterAssistantAct = nullptr;

// New cluster spot → score it → feed to assistant
connect(dxClusterWidget.get(), &DXClusterWidget::dxspotArrived,
        this, &MainWindow::slotDXAssistantNewSpot);

// New QSO logged → recalculate all spot scores
connect(&awards, &Awards::awardDXCCUpdated,
        this, &MainWindow::slotDXAssistantRecalculate);

// ClubLog most-wanted list refreshed → recalculate
connect(clubLogMostWanted, &ClubLogMostWanted::mostWantedUpdated,
        this, &MainWindow::slotDXAssistantRecalculate);

// User activates a spot → tune radio + fill QSO entry form
connect(dxClusterAssistant.get(), &DXClusterAssistant::spotTuneRequested,
        this, &MainWindow::slotDXAssistantSpotTuneRequested);

// Called for every arriving DXCluster spot
void MainWindow::slotDXAssistantNewSpot(const DXSpot &spot)
{
    if (!dxClusterAssistant) return;
    DXAssistantSpot scored = dxAssistantEngine->score(spot);
    if (scored.score >= 0)   // score == -1 means discard (confirmed / invalid)
        dxClusterAssistant->addOrUpdateSpot(scored);
}

// Called after Awards update or ClubLog list refresh
void MainWindow::slotDXAssistantRecalculate()
{
    if (dxClusterAssistant)
        dxClusterAssistant->recalculateAll();
}

// Called when user double-clicks or context-menu "tunes" to a spot
void MainWindow::slotDXAssistantSpotTuneRequested(const DXAssistantSpot &spot)
{
    clusterSpotToLog(spot.dxCall, spot.freq);
}

// Lazy show/hide
void MainWindow::slotShowDXClusterAssistant()
{
    if (!dxClusterAssistant) {
        dxClusterAssistant = std::make_unique<DXClusterAssistant>(
            &awards, world, dataProxy, Q_FUNC_INFO, this);
        dxClusterAssistant->init();
        // re-connect spotTuneRequested here
    }
    dxClusterAssistant->show();
    dxClusterAssistant->raise();
}

dxClusterAssistantAct = new QAction(tr("DX Assistant"), this);
dxClusterAssistantAct->setToolTip(tr("Show the DX Assistant — prioritised list of workable spots."));
toolMenu->addAction(dxClusterAssistantAct);
connect(dxClusterAssistantAct, &QAction::triggered,
        this, &MainWindow::slotShowDXClusterAssistant);