Implement duel request/accept/decline UI and packet handling

- Parse SMSG_DUEL_REQUESTED: store challenger guid/name, set
  pendingDuelRequest_ flag, show chat notification
- Parse SMSG_DUEL_COMPLETE: clear pending flag, notify on cancel
- Parse SMSG_DUEL_WINNER: show "X defeated Y in a duel!" chat message
- Handle SMSG_DUEL_OUTOFBOUNDS with warning message
- Add acceptDuel() method sending CMSG_DUEL_ACCEPTED (new builder)
- Wire forfeitDuel() to clear pendingDuelRequest_ on decline
- Add renderDuelRequestPopup() ImGui window (Accept/Decline buttons)
  positioned near group invite popup; shown when challenge is pending
- Add DuelAcceptPacket builder to world_packets.hpp/cpp

include/game/world_packets.hpp

@@ -1265,6 +1265,12 @@ public:
 // Duel
 // ============================================================
 
+/** CMSG_DUEL_ACCEPTED packet builder (no payload) */
+class DuelAcceptPacket {
+public:
+    static network::Packet build();
+};
+
 /** CMSG_DUEL_CANCELLED packet builder */
 class DuelCancelPacket {
 public: