viewer: give the current-image plot marker a fixed colour

The current-image marker (a scatter series) was picking an arbitrary theme colour
(green), which read as a stray series. Pin it to black so it's an unambiguous
"current image" dot, distinct from the run line colours. Its legend entry is
already hidden.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

viewer/charts/JFJochDatasetInfoChartView.cpp

@@ -228,6 +228,8 @@ void JFJochDatasetInfoChartView::buildTimeDomainChart() {
         if (primary_color_.isValid())
             series->setColor(primary_color_);
         currentSeries = new QScatterSeries(this);
+        currentSeries->setColor(Qt::black);     // "current image" marker: fixed, not a run colour
+        currentSeries->setMarkerSize(9.0);
 
         double dispMin = std::numeric_limits<double>::infinity();
         double dispMax = -std::numeric_limits<double>::infinity();