Plot tab: axis-selective wheel zoom (X default, Shift+wheel for Y)

Add PlotViewBox subclassing pg.ViewBox. Overrides wheelEvent to route
plain scroll to the X axis and Shift+scroll to the Y axis, both centred
on the cursor position. Passed as viewBox= to every PlotWidget so the
behaviour applies uniformly across all 8 plots.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gui_client.py

@@ -390,6 +390,16 @@ class TimeAxisItem(pg.AxisItem):
         return [datetime.fromtimestamp(v).strftime("%H:%M:%S") for v in values]
 
 
+class PlotViewBox(pg.ViewBox):
+    """ViewBox with axis-selective wheel zoom: plain wheel → X, Shift+wheel → Y."""
+    def wheelEvent(self, ev, axis=None):
+        if ev.modifiers() & Qt.ShiftModifier:
+            axis = 1   # Y only
+        else:
+            axis = 0   # X only
+        super().wheelEvent(ev, axis=axis)
+
+
 class PlotTooltip(QWidget):
     """Floating value readout that follows the crosshair cursor."""
     def __init__(self, parent: QWidget):
@@ -523,6 +533,7 @@ class PlotTab(QWidget):
             pw = pg.PlotWidget(
                 background="#12121e",
                 axisItems={"bottom": TimeAxisItem(orientation="bottom")},
+                viewBox=PlotViewBox(),
             )
             pw.showGrid(x=True, y=True, alpha=0.3)
             pw.addLegend(offset=(0, 0), labelTextSize="8pt")