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);
+}