refactor: Introduce ApplicationController to centralize logic

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

src/MainWindow.h

@@ -24,12 +24,12 @@ private slots:
     void createSketch();
     void saveSketch();
 
-private:
-    void setCurrentFile(const QString &fileName);
+    void enterSketchMode();
+    void exitSketchMode();
+    void updateWindowTitle(const QString& filePath);
 
+private:
     ViewportWidget *m_viewport;
-    Document *m_document;
-    QString m_currentFile;
 
     QTabWidget *m_tabWidget;
     QWidget *m_solidTab;