Implement the REQ-UI-MULTI-SELECTION addition: the selected building panel now shows the total placement cost of a multi-selection, counting only player-placeable buildings (HQ and defence stations excluded). Construction sites are charged their type's full cost. Add a shared BuildingsConfig::findBuildingDef accessor and reuse it in BlueprintPanel to remove the duplicated by-type lookup loops. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DZR44tA8sn4dPqDzAVXyps