feat: show quality-coloured loot toast when items are received

SMSG_ITEM_PUSH_RESULT now fires a new ItemLootCallback that
game_screen.cpp uses to push a compact slide-in toast at the
bottom-left of the screen.  Each toast:

- Shows a quality-tinted left accent bar (grey/white/green/blue/
  purple/orange matching WoW quality colours)
- Displays "Loot: <item name>" with the name in quality colour
- Appends " x<N>" for stacked pickups
- Coalesces repeated pickups of the same item (adds count, resets timer)
- Stacks up to 5 entries, 3 s lifetime with 0.15 s slide-in and 0.7 s
  fade-out

src/game/game_handler.cpp

@@ -1701,12 +1701,17 @@ void GameHandler::handlePacket(network::Packet& packet) {
                 queryItemInfo(itemId, 0);
                 if (showInChat) {
                     std::string itemName = "item #" + std::to_string(itemId);
+                    uint32_t quality = 1;  // white default
                     if (const ItemQueryResponseData* info = getItemInfo(itemId)) {
                         if (!info->name.empty()) itemName = info->name;
+                        quality = info->quality;
                     }
                     std::string msg = "Received: " + itemName;
                     if (count > 1) msg += " x" + std::to_string(count);
                     addSystemChatMessage(msg);
+                    if (itemLootCallback_) {
+                        itemLootCallback_(itemId, count, quality, itemName);
+                    }
                 }
                 LOG_INFO("Item push: itemId=", itemId, " count=", count,
                          " showInChat=", static_cast<int>(showInChat));