feat: add failing tests for MainWindow display behaviour

tests/test_main_window.cxx

@@ -0,0 +1,47 @@
+#include <gtest/gtest.h>
+
+#include <QApplication>
+#include <QLabel>
+#include <QSignalSpy>
+#include <cstdlib>
+
+#include "MainWindow.hpp"
+
+// QWidget-based tests need a full QApplication (not QCoreApplication).
+// Use offscreen platform so tests run headless in containers.
+static int argc_ = 0;
+static char* argv_[] = {nullptr};
+static struct SetupOffscreen
+{
+  SetupOffscreen() { qputenv("QT_QPA_PLATFORM", "offscreen"); }
+} setup_offscreen_;
+static QApplication app_(argc_, argv_);
+
+TEST(MainWindowTest, LabelUpdatesOnValueReceived)
+{
+  MainWindow window;
+  // Simulate receiving a value from ConsumerThread
+  window.onValueReceived(42);
+
+  // The label should display the received value
+  EXPECT_NE(window.lastDisplayedText().toStdString().find("42"),
+            std::string::npos);
+}
+
+TEST(MainWindowTest, LabelUpdatesMultipleTimes)
+{
+  MainWindow window;
+  window.onValueReceived(10);
+  window.onValueReceived(20);
+  window.onValueReceived(30);
+
+  // Label should show the most recent value
+  EXPECT_NE(window.lastDisplayedText().toStdString().find("30"),
+            std::string::npos);
+}
+
+TEST(MainWindowTest, WindowTitleIsSet)
+{
+  MainWindow window;
+  EXPECT_FALSE(window.windowTitle().isEmpty());
+}