plan: define MainWindow Qt widget interface

include/MainWindow.hpp

@@ -0,0 +1,32 @@
+#pragma once
+// MainWindow.hpp
+// SPDX-License-Identifier: GPL-3.0-only
+// Author: Unai Blazquez <unaibg2000@gmail.com>
+
+#include <QLabel>
+#include <QString>
+#include <QVBoxLayout>
+#include <QWidget>
+
+/// @brief Minimal GUI window that displays the last integer received
+///        from the ConsumerThread. Never blocks — values arrive via
+///        Qt's queued signal/slot mechanism.
+class MainWindow : public QWidget
+{
+  Q_OBJECT
+
+ public:
+  explicit MainWindow(QWidget* parent = nullptr);
+
+  /// @brief Returns the current text shown in the value label (for testing).
+  QString lastDisplayedText() const;
+
+ public slots:
+  /// @brief Slot connected to ConsumerThread::valueReceived.
+  ///        Updates the label with the new value.
+  void onValueReceived(int value);
+
+ private:
+  QLabel* m_title_label;
+  QLabel* m_value_label;
+};