feat: Implement main window with tool banner and viewport placeholder

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

src/MainWindow.cpp

@@ -0,0 +1,17 @@
+#include "MainWindow.h"
+
+#include <QToolBar>
+#include <QWidget>
+
+MainWindow::MainWindow(QWidget *parent)
+    : QMainWindow(parent)
+{
+    setWindowTitle("OpenCAD");
+    resize(800, 600);
+
+    QToolBar *toolBar = addToolBar("Tools");
+
+    QWidget *viewport = new QWidget;
+    viewport->setStyleSheet("background-color: darkgray;");
+    setCentralWidget(viewport);
+}

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

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();