feat: Render view cube labels as OpenGL textures

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

src/ViewportWidget.h

@@ -26,8 +26,10 @@ private:
     void drawCube();
     void drawViewCube();
     void drawAxes();
+    void createFaceTextures();
 
     QMatrix4x4 projection;
+    GLuint faceTextures[6];
 
     float xRot = 0;
     float yRot = 0;