feat: Implement main window with tool banner and viewport placeholder

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

src/MainWindow.h

@@ -0,0 +1,14 @@
+#ifndef MAINWINDOW_H
+#define MAINWINDOW_H
+
+#include <QMainWindow>
+
+class MainWindow : public QMainWindow
+{
+    Q_OBJECT
+
+public:
+    explicit MainWindow(QWidget *parent = nullptr);
+};
+
+#endif // MAINWINDOW_H