feat: Add debug window; accessible via Help menu and DEBUG env var

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

src/MainWindow.h

@@ -10,6 +10,7 @@ class Feature;
 class QTabWidget;
 class QWidget;
 class ApplicationController;
+class DebugWindow;
 
 class MainWindow : public QMainWindow
 {
@@ -26,6 +27,8 @@ private slots:
     void createSketch();
     void saveSketch();
 
+    void showDebugWindow();
+
     void enterSketchMode();
     void exitSketchMode();
     void updateWindowTitle(const QString& filePath);
@@ -36,6 +39,7 @@ protected:
 private:
     ApplicationController* m_appController;
     ViewportWidget *m_viewport;
+    DebugWindow* m_debugWindow;
 
     QTabWidget *m_tabWidget;
     QWidget *m_solidTab;