make HeaderBar read block stock and expansion cost from the simulation

HeaderBar was the only panel caching event payloads as truth. It now holds
a const Simulation* and re-reads getBuildingBlocksStock() /
getCurrentExpansionCost() in the handlers, per the "events are refresh
signals" rule.

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

src/ui/MainWindow.cpp

@@ -47,7 +47,7 @@ MainWindow::MainWindow(Simulation* sim, const std::string& configDir,
     const std::string itemsIconDir = QDir::cleanPath(
         QString::fromStdString(m_configDir) + "/../icons/items").toStdString();
 
-    m_headerBar = new HeaderBar(&sim->getConfig(), itemsIconDir, this);
+    m_headerBar = new HeaderBar(sim, &sim->getConfig(), itemsIconDir, this);
 
     m_gameWorldView = new GameWorldView(sim, &sim->getConfig(), &m_visuals, m_configDir,
                                         m_replay.get(), this);