feat: Implement feature graph and JSON document save/load

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

src/MainWindow.h

@@ -4,6 +4,7 @@
 #include <QMainWindow>
 
 class ViewportWidget;
+class Document;
 
 class MainWindow : public QMainWindow
 {
@@ -13,10 +14,18 @@ public:
     explicit MainWindow(QWidget *parent = nullptr);
 
 private slots:
+    void newFile();
+    void open();
+    bool save();
+    bool saveAs();
     void createSketch();
 
 private:
+    void setCurrentFile(const QString &fileName);
+
     ViewportWidget *m_viewport;
+    Document *m_document;
+    QString m_currentFile;
 };
 
 #endif // MAINWINDOW_H