feat: Implement main window with tool banner and viewport placeholder

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

src/main.cpp

@@ -1,13 +1,11 @@
 #include <QApplication>
-#include <QMainWindow>
+#include "MainWindow.h"
 
 int main(int argc, char *argv[])
 {
     QApplication app(argc, argv);
 
-    QMainWindow window;
-    window.setWindowTitle("OpenCAD");
-    window.resize(800, 600);
+    MainWindow window;
     window.show();
 
     return app.exec();