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/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: