refactor: Introduce ApplicationController to centralize logic

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

src/ApplicationController.h

@@ -0,0 +1,46 @@
+#ifndef APPLICATIONCONTROLLER_H
+#define APPLICATIONCONTROLLER_H
+
+#include <QObject>
+#include "ViewportWidget.h" // For SketchPlane enum
+
+class Document;
+class MainWindow;
+
+class ApplicationController : public QObject
+{
+    Q_OBJECT
+public:
+    static ApplicationController* instance();
+
+    void setMainWindow(MainWindow* mainWindow);
+    Document* document() const;
+
+public slots:
+    void newDocument();
+    bool openDocument();
+    bool saveDocument();
+    bool saveDocumentAs();
+
+    void beginSketchCreation();
+    void endSketch();
+
+signals:
+    void sketchModeStarted(ViewportWidget::SketchPlane plane);
+    void sketchModeEnded();
+    void currentFileChanged(const QString& path);
+
+private:
+    explicit ApplicationController(QObject *parent = nullptr);
+    ~ApplicationController();
+    ApplicationController(const ApplicationController&) = delete;
+    ApplicationController& operator=(const ApplicationController&) = delete;
+
+    void setCurrentFile(const QString& fileName);
+
+    Document* m_document;
+    QString m_currentFile;
+    MainWindow* m_mainWindow = nullptr;
+};
+
+#endif // APPLICATIONCONTROLLER_H