Allow to draw produced-item icons in recipe dialog and game world

src/lib/config/RecipesConfig.h

@@ -32,6 +32,10 @@ struct RecipeDef
     std::vector<RecipeIngredient> inputs;
     std::vector<RecipeOutput> outputs;
     double durationSeconds;
+    // Optional id of the item whose icon represents this recipe in the recipe-
+    // selection dialog (REQ-UI-RECIPE-ICON). When unset, the first output item is
+    // used. A missing icon file for that item is not an error (REQ-UI-ITEM-ICON).
+    std::optional<std::string> icon;
     // Assembler only. When true, this recipe is available from game start
     // regardless of the implicit item graph — used for base recipes that no
     // schematic's materials reach (e.g. building blocks). See REQ-LOCK-IMPLICIT.