viewer: keep plot axis labels readable when compressed

When the plot dock was short, Qt Charts dropped the axis labels first, making
the plot hard to read.

- Reclaim Qt Charts' outer graphics-layout padding and trim the inner margins
  (both chart views) so the plot and its labels get the available space.
- Raise the chart minimum height (dataset-info 80 -> 140, per-image 120 -> 140)
  so the dock can't be squeezed below the point where the axis labels fit.

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

viewer/charts/JFJochSimpleChartView.cpp

@@ -8,11 +8,14 @@
 #include <QApplication>
 #include <QCategoryAxis>
 #include <QRegularExpression>
+#include <QGraphicsLayout>
 
 JFJochSimpleChartView::JFJochSimpleChartView(QWidget *parent)
     : QChartView(new QChart(), parent) {
     chart()->legend()->hide();
-    setMinimumHeight(120);   // soft floor: let the host dock/window decide the height
+    chart()->layout()->setContentsMargins(0, 0, 0, 0);   // reclaim padding for the plot + labels
+    chart()->setMargins(QMargins(2, 2, 6, 2));
+    setMinimumHeight(140);   // soft floor: enough for the axis labels to stay readable
     setRenderHint(QPainter::Antialiasing);
     setMouseTracking(true);
     //setRubberBand(QChartView::RubberBand::HorizontalRubberBand);