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

src/ui/ShipLayoutPreview.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <optional>
 #include <string>
 #include <vector>
 
@@ -31,7 +32,7 @@ private:
     struct CellInfo
     {
         bool buildable;
-        int moduleIndex; // -1 if empty
+        std::optional<int> moduleIndex;  // nullopt if empty
     };
 
     std::vector<std::vector<CellInfo>> m_grid;