refactor: Migrate ViewportWidget to shader-based OpenGL rendering (FFP disabled)

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

src/ViewportWidget.h

@@ -9,6 +9,9 @@
 #include <QRect>
 #include <gp_Pnt.hxx>
 #include <QMap>
+#include <QOpenGLShaderProgram>
+#include <QOpenGLVertexArrayObject>
+#include <QOpenGLBuffer>
 
 class QSvgRenderer;
 class ViewCube;
@@ -56,6 +59,7 @@ protected:
     void keyPressEvent(QKeyEvent *event) override;
 
 private:
+    void initShaders();
     QVector3D project(const QVector3D& worldCoord, const QMatrix4x4& modelView, const QMatrix4x4& projection, const QRect& viewport);
     QVector3D unproject(const QPoint& screenPos, SketchPlane plane);
     void drawAxisLabels(QPainter& painter, const QMatrix4x4& modelView, const QMatrix4x4& projection);
@@ -64,6 +68,14 @@ private:
     ViewportWidget::SketchPlane checkPlaneSelection(const QPoint& screenPos);
 
     QMatrix4x4 projection;
+    QOpenGLShaderProgram* m_shaderProgram = nullptr;
+    QOpenGLVertexArrayObject m_vao;
+    QOpenGLBuffer m_vbo;
+
+    // Shader uniform locations
+    int m_projMatrixLoc = -1;
+    int m_mvMatrixLoc = -1;
+    int m_colorLoc = -1;
     Camera* m_camera = nullptr;
     ViewCube* m_viewCube;
     SketchGrid* m_sketchGrid = nullptr;