feat: implement trade window UI with item slots and gold offering

Previously trade only showed an accept/decline popup with no way to
actually offer items or gold. This commit adds the complete trade flow:

Packets:
- CMSG_SET_TRADE_ITEM (tradeSlot, bag, bagSlot) — add item to slot
- CMSG_CLEAR_TRADE_ITEM (tradeSlot) — remove item from slot
- CMSG_SET_TRADE_GOLD (uint64 copper) — set gold offered
- CMSG_UNACCEPT_TRADE — unaccept without cancelling
- SMSG_TRADE_STATUS_EXTENDED parser — updates trade slot/gold state

State:
- TradeSlot struct: itemId, displayId, stackCount, bag, bagSlot
- myTradeSlots_/peerTradeSlots_ arrays (6 slots each)
- myTradeGold_/peerTradeGold_ (copper)
- resetTradeState() helper clears all state on cancel/complete/close

UI (renderTradeWindow):
- Two-column layout: my offer | peer offer
- Each column shows 6 item slots with item names
- Double-click own slot to remove; right-click empty slot to open
  backpack picker popup
- Gold input field (copper, Enter to set)
- Accept Trade / Cancel buttons
- Window close button triggers cancel trade

include/game/game_handler.hpp

@@ -933,13 +933,38 @@ public:
     enum class TradeStatus : uint8_t {
         None = 0, PendingIncoming, Open, Accepted, Complete
     };
+
+    static constexpr int TRADE_SLOT_COUNT = 6;  // WoW has 6 normal trade slots + slot 6 for non-trade item
+
+    struct TradeSlot {
+        uint32_t itemId      = 0;
+        uint32_t displayId   = 0;
+        uint32_t stackCount  = 0;
+        uint64_t itemGuid    = 0;
+        uint8_t  bag         = 0xFF;   // 0xFF = not set
+        uint8_t  bagSlot     = 0xFF;
+        bool     occupied    = false;
+    };
+
     TradeStatus getTradeStatus() const { return tradeStatus_; }
     bool hasPendingTradeRequest() const { return tradeStatus_ == TradeStatus::PendingIncoming; }
+    bool isTradeOpen() const { return tradeStatus_ == TradeStatus::Open || tradeStatus_ == TradeStatus::Accepted; }
     const std::string& getTradePeerName() const { return tradePeerName_; }
+
+    // My trade slots (what I'm offering)
+    const std::array<TradeSlot, TRADE_SLOT_COUNT>& getMyTradeSlots() const { return myTradeSlots_; }
+    // Peer's trade slots (what they're offering)
+    const std::array<TradeSlot, TRADE_SLOT_COUNT>& getPeerTradeSlots() const { return peerTradeSlots_; }
+    uint64_t getMyTradeGold() const { return myTradeGold_; }
+    uint64_t getPeerTradeGold() const { return peerTradeGold_; }
+
     void acceptTradeRequest();   // respond to incoming SMSG_TRADE_STATUS(1) with CMSG_BEGIN_TRADE
     void declineTradeRequest();  // respond with CMSG_CANCEL_TRADE
     void acceptTrade();          // lock in offer: CMSG_ACCEPT_TRADE
     void cancelTrade();          // CMSG_CANCEL_TRADE
+    void setTradeItem(uint8_t tradeSlot, uint8_t bag, uint8_t bagSlot);
+    void clearTradeItem(uint8_t tradeSlot);
+    void setTradeGold(uint64_t copper);
 
     // ---- Duel ----
     bool hasPendingDuelRequest() const { return pendingDuelRequest_; }
@@ -1653,6 +1678,8 @@ private:
     void handleQuestConfirmAccept(network::Packet& packet);
     void handleSummonRequest(network::Packet& packet);
     void handleTradeStatus(network::Packet& packet);
+    void handleTradeStatusExtended(network::Packet& packet);
+    void resetTradeState();
     void handleDuelRequested(network::Packet& packet);
     void handleDuelComplete(network::Packet& packet);
     void handleDuelWinner(network::Packet& packet);
@@ -2077,6 +2104,10 @@ private:
     TradeStatus tradeStatus_  = TradeStatus::None;
     uint64_t    tradePeerGuid_= 0;
     std::string tradePeerName_;
+    std::array<TradeSlot, TRADE_SLOT_COUNT> myTradeSlots_{};
+    std::array<TradeSlot, TRADE_SLOT_COUNT> peerTradeSlots_{};
+    uint64_t myTradeGold_   = 0;
+    uint64_t peerTradeGold_ = 0;
 
     // Duel state
     bool pendingDuelRequest_    = false;

include/game/world_packets.hpp

@@ -1356,6 +1356,33 @@ public:
     static network::Packet build();
 };
 
+/** CMSG_SET_TRADE_ITEM packet builder (tradeSlot, bag, bagSlot) */
+class SetTradeItemPacket {
+public:
+    // tradeSlot: 0-5 (normal) or 6 (backpack money-only slot)
+    // bag: 255 = main backpack, 19-22 = bag slots
+    // bagSlot: slot within bag
+    static network::Packet build(uint8_t tradeSlot, uint8_t bag, uint8_t bagSlot);
+};
+
+/** CMSG_CLEAR_TRADE_ITEM packet builder (remove item from trade slot) */
+class ClearTradeItemPacket {
+public:
+    static network::Packet build(uint8_t tradeSlot);
+};
+
+/** CMSG_SET_TRADE_GOLD packet builder (gold offered, in copper) */
+class SetTradeGoldPacket {
+public:
+    static network::Packet build(uint64_t copper);
+};
+
+/** CMSG_UNACCEPT_TRADE packet builder (unaccept without cancelling) */
+class UnacceptTradePacket {
+public:
+    static network::Packet build();
+};
+
 /** CMSG_ATTACKSWING packet builder */
 class AttackSwingPacket {
 public:

include/ui/game_screen.hpp

@@ -224,6 +224,7 @@ private:
     void renderDuelRequestPopup(game::GameHandler& gameHandler);
     void renderLootRollPopup(game::GameHandler& gameHandler);
     void renderTradeRequestPopup(game::GameHandler& gameHandler);
+    void renderTradeWindow(game::GameHandler& gameHandler);
     void renderSummonRequestPopup(game::GameHandler& gameHandler);
     void renderSharedQuestPopup(game::GameHandler& gameHandler);
     void renderItemTextWindow(game::GameHandler& gameHandler);