feat: add DemoApp and ShutdownApp classes for interactive UI components and shutdown management

main/ui/apps/shutdown_app.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include "ui/ui_app.h"
+#include "ui/app_registry.h"
+
+/**
+ * @brief Shutdown application - displays shutdown message
+ *
+ * Shown when the system is about to enter deep sleep or power off.
+ * Displays a message and optionally a spinner animation.
+ */
+class ShutdownApp : public UIApp {
+public:
+  ShutdownApp(std::string message = "");
+  virtual ~ShutdownApp() = default;
+
+  esp_err_t init(lv_obj_t* container) override;
+  esp_err_t deinit(void) override;
+  std::string get_name(void) const override;
+
+private:
+  std::string _message;
+  lv_obj_t* _label_message = nullptr;
+};
+
+/**
+ * @brief AppDescriptor for ShutdownApp
+ *
+ * Note: Shutdown app is typically not shown in the navigation bar
+ * as it's only used during system shutdown.
+ */
+class ShutdownAppDescriptor : public AppDescriptor {
+public:
+  ShutdownAppDescriptor();
+  void draw_icon(lv_obj_t* parent) override;
+
+private:
+  static ShutdownApp* _app_instance;
+};