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.cpp

@@ -184,13 +184,12 @@ namespace
 
 BuildButtonGrid::BuildButtonGrid(Simulation* sim, const GameConfig* config,
                                  const std::string& iconDir,
-                                 const std::string& itemsIconDir, QWidget* parent)
+                                 ItemIconCache* itemIcons, QWidget* parent)
     : QWidget(parent)
     , m_sim(sim)
     , m_config(config)
     , m_iconDir(iconDir)
-    , m_itemIcons(std::make_unique<ItemIconCache>(
-          QString::fromStdString(itemsIconDir)))
+    , m_itemIcons(itemIcons)
 {
     QGridLayout* layout = new QGridLayout(this);
     layout->setSpacing(4);