Use std::optional instead of sentinel values for absent data

src/ui/BlueprintPanel.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <optional>
 #include <vector>
 
 #include <QWidget>
@@ -56,7 +57,7 @@ private:
     const GameConfig*         m_config;
     std::vector<BuildingId>     m_selectedBuildingIds;
     int                       m_currentBlocks;
-    int                       m_activeIndex;
+    std::optional<int>        m_activeIndex;  // nullopt = no blueprint selected
     std::vector<Blueprint>    m_blueprints;
     std::vector<QPushButton*> m_blueprintButtons;
     QPushButton*              m_createBtn;