refactor: Replace ApplicationController singleton with dependency injection

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

src/MainWindow.h

@@ -8,13 +8,14 @@ class Document;
 class Feature;
 class QTabWidget;
 class QWidget;
+class ApplicationController;
 
 class MainWindow : public QMainWindow
 {
     Q_OBJECT
 
 public:
-    explicit MainWindow(QWidget *parent = nullptr);
+    explicit MainWindow(ApplicationController* appController, QWidget *parent = nullptr);
 
 private slots:
     void newFile();
@@ -29,6 +30,7 @@ private slots:
     void updateWindowTitle(const QString& filePath);
 
 private:
+    ApplicationController* m_appController;
     ViewportWidget *m_viewport;
 
     QTabWidget *m_tabWidget;