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

include/game/game_handler.hpp

@@ -1494,6 +1494,10 @@ public:
     using PvpHonorCallback = std::function<void(uint32_t honorAmount, uint64_t victimGuid, uint32_t victimRank)>;
     void setPvpHonorCallback(PvpHonorCallback cb) { pvpHonorCallback_ = std::move(cb); }
 
+    // Item looted / received callback (SMSG_ITEM_PUSH_RESULT when showInChat is set)
+    using ItemLootCallback = std::function<void(uint32_t itemId, uint32_t count, uint32_t quality, const std::string& name)>;
+    void setItemLootCallback(ItemLootCallback cb) { itemLootCallback_ = std::move(cb); }
+
     // Quest turn-in completion callback
     using QuestCompleteCallback = std::function<void(uint32_t questId, const std::string& questTitle)>;
     void setQuestCompleteCallback(QuestCompleteCallback cb) { questCompleteCallback_ = std::move(cb); }
@@ -2838,6 +2842,9 @@ private:
     // ---- PvP honor credit callback ----
     PvpHonorCallback pvpHonorCallback_;
 
+    // ---- Item loot callback ----
+    ItemLootCallback itemLootCallback_;
+
     // ---- Quest completion callback ----
     QuestCompleteCallback questCompleteCallback_;
 };