feat: Add feature browser dock widget with document synchronization

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

src/MainWindow.h

@@ -5,6 +5,9 @@
 
 class ViewportWidget;
 class Document;
+class Feature;
+class QTreeWidget;
+class QTreeWidgetItem;
 
 class MainWindow : public QMainWindow
 {
@@ -20,12 +23,18 @@ private slots:
     bool saveAs();
     void createSketch();
 
+    void onFeatureAdded(Feature* feature);
+    void onDocumentCleared();
+
 private:
     void setCurrentFile(const QString &fileName);
 
     ViewportWidget *m_viewport;
     Document *m_document;
     QString m_currentFile;
+
+    QTreeWidget* m_featureTree;
+    QTreeWidgetItem* m_rootItem;
 };
 
 #endif // MAINWINDOW_H