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/BuildButtonGrid.h

@@ -32,11 +32,12 @@ class BuildButtonGrid : public QWidget,
 
 public:
     // iconDir is the directory holding the per-building "<id>.svg" chip icons
-    // (REQ-UI-BUILD-GRID); itemsIconDir holds the per-item icons and supplies the
-    // building_block icon shown in each button's cost (REQ-UI-BUILD-COST). Both are
-    // read from disk at runtime, like the config files.
+    // (REQ-UI-BUILD-GRID), read from disk at runtime like the config files.
+    // itemIcons is the window-wide per-item icon cache and supplies the
+    // building_block icon shown in each button's cost (REQ-UI-BUILD-COST). Not
+    // owned; must outlive this widget.
     BuildButtonGrid(Simulation* sim, const GameConfig* config,
-                    const std::string& iconDir, const std::string& itemsIconDir,
+                    const std::string& iconDir, ItemIconCache* itemIcons,
                     QWidget* parent = nullptr);
     ~BuildButtonGrid() override;
 
@@ -65,7 +66,7 @@ private:
     Simulation*                      m_sim;
     const GameConfig*                m_config;
     std::string                      m_iconDir;
-    std::unique_ptr<ItemIconCache>   m_itemIcons;
+    ItemIconCache*                   m_itemIcons;  // Not owned; lives in MainWindow.
     std::vector<BuildingType>        m_types;
     std::vector<QPushButton*>        m_buttons;
     std::map<BuildingType, int>      m_costs;