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/HeaderBar.cpp

@@ -32,10 +32,9 @@ const double HeaderBar::kSpeeds[]   = { 0.0, 0.5, 1.0, 2.0, 10.0 };
 const int    HeaderBar::kSpeedCount = 5;
 
 HeaderBar::HeaderBar(const Simulation* sim, const GameConfig* config,
-                     const std::string& itemsIconDir, QWidget* parent)
+                     ItemIconCache* itemIcons, QWidget* parent)
     : QWidget(parent)
-    , m_itemIcons(std::make_unique<ItemIconCache>(
-          QString::fromStdString(itemsIconDir)))
+    , m_itemIcons(itemIcons)
     , m_sim(sim)
 {
     QHBoxLayout* layout = new QHBoxLayout(this);