Closes #4111

What this PR does

Adds logarithmic scale support for the X and Y axes in the Plot window,
accessible via two new options in the plot's right-click context menu:

• "Logarithmic scale (X)"
• "Logarithmic scale (Y)"

Implementation

Follows the same pattern as existing toggleable plot options
(Show legend, Stacked bars, Fixed number format):

• Added m_logScaleX and m_logScaleY bool members to PlotDock
• Added updateLogScale() function that calls QCPAxis::setScaleType()
  on xAxis, yAxis, and yAxis2
• Added two checkable QAction entries to the context menu
• Called updateLogScale() after rescaleAxes() in updatePlot() so
  the scale type is reapplied whenever the plot data changes
• Guards against log(0): clamps axis range minimum to 1e-3 when
  switching to log scale with a non-positive lower bound

How to test

1. Open a database with numeric data
2. Go to Browse Data → Plot tab
3. Select a numeric column as X and another as Y1
4. Right-click the plot → toggle "Logarithmic scale (Y)"
5. Y axis switches to logarithmic spacing
6. Toggle off → returns to linear
7. Same works for "Logarithmic scale (X)"