feat: Display sketch grid preview on plane highlight

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

src/ViewportWidget.cpp

@@ -136,6 +136,9 @@ void ViewportWidget::paintGL()
     glLoadMatrixf(model.constData());
 
     if (m_isSelectingPlane) {
+        if (m_highlightedPlane != SketchPlane::NONE) {
+            m_sketchGrid->paintGL(static_cast<SketchGrid::SketchPlane>(m_highlightedPlane), projection, model);
+        }
         drawSelectionPlanes();
     }