share a single ItemIconCache across the UI

Four separate caches rasterized the same item SVGs, one of them rebuilt on
every recipe-dialog open. MainWindow now owns one cache and hands a
non-owning pointer to HeaderBar, BuildButtonGrid, GameWorldView and
RecipeSelectionDialog.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GH8ZMRY3vhxxXcaUxBqxkk

src/ui/MainWindow.h

@@ -27,6 +27,7 @@ class HeaderBar;
 class SelectedBuildingPanel;
 class BuildButtonGrid;
 class BlueprintPanel;
+class ItemIconCache;
 class QCloseEvent;
 class QResizeEvent;
 
@@ -68,6 +69,9 @@ private:
     std::string            m_configDir;
     VisualsConfig          m_visuals;
     Simulation*            m_sim;
+    // One per-item icon cache for the whole window (REQ-UI-ITEM-ICON): the header,
+    // build grid, world view, and recipe dialog all rasterize the same SVGs.
+    std::unique_ptr<ItemIconCache> m_itemIcons;
     GameWorldView*         m_gameWorldView;
     HeaderBar*             m_headerBar;
     SelectedBuildingPanel* m_selectedBuildingPanel;