Fix compilation errors from single-player removal

- Fixed corrupted header (removed orphaned code fragment)
- Restored NPC callbacks needed for online animations
  - NpcDeathCallback, NpcRespawnCallback, NpcSwingCallback
  - These were incorrectly removed as "SP-only" but are used for animations in online mode
- Removed calls to deleted methods:
  - getItemTemplateName, getItemTemplateQuality (used fallback in loot window)
  - notifyInventoryChanged, notifyEquipmentChanged (SP persistence markers)
- Removed hearthstone single-player handling (now uses server)

All online features preserved. Code should now compile.

src/ui/inventory_screen.cpp

@@ -628,9 +628,6 @@ void InventoryScreen::render(game::Inventory& inventory, uint64_t moneyCopper) {
             heldItem = game::ItemDef{};
             heldSource = HeldSource::NONE;
             inventoryDirty = true;
-            if (gameHandler_) {
-                gameHandler_->notifyInventoryChanged();
-            }
             dropItemName_.clear();
             ImGui::CloseCurrentPopup();
         }