feat: Add Blinn-Phong shading for faces

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

src/ViewportWidget.h

@@ -97,6 +97,7 @@ private:
 
     QMatrix4x4 projection;
     QOpenGLShaderProgram* m_shaderProgram = nullptr;
+    QOpenGLShaderProgram* m_litShaderProgram = nullptr;
     QOpenGLVertexArrayObject m_vao;
     QOpenGLBuffer m_vbo;
 
@@ -104,6 +105,11 @@ private:
     int m_projMatrixLoc = -1;
     int m_mvMatrixLoc = -1;
     int m_colorLoc = -1;
+
+    // Lit shader uniform locations
+    int m_litProjMatrixLoc = -1;
+    int m_litMvMatrixLoc = -1;
+    int m_litNormalMatrixLoc = -1;
     Camera* m_camera = nullptr;
     ViewCube* m_viewCube;
     SketchGrid* m_sketchGrid = nullptr;