Add Tier 7 commands: combat and trade

Combat Commands:
- /duel - Challenge target to a duel (CMSG_DUEL_PROPOSED 0x166)
- /trade - Open trade window with target (CMSG_INITIATE_TRADE 0x116)
- /startattack - Begin auto-attacking target
- /stopattack - Stop auto-attacking
- /stopcasting - Cancel current spell cast

New opcodes:
- CMSG_DUEL_PROPOSED (0x166) for initiating duels
- CMSG_INITIATE_TRADE (0x116) for starting trades

Packet builders:
- DuelProposedPacket - sends duel challenge to target GUID
- InitiateTradePacket - sends trade request to target GUID
- AttackSwingPacket, AttackStopPacket, CancelCastPacket reused from existing

Game handler methods:
- proposeDuel(targetGuid) - challenge target to duel
- initiateTrade(targetGuid) - open trade with target
- stopCasting() - cancel current spell cast (uses existing casting state)

All commands include validation for target selection and world state.
Removed duplicate packet class definitions from previous phases.

include/game/game_handler.hpp

@@ -270,6 +270,11 @@ public:
     void clearMainAssist();
     void requestRaidInfo();
 
+    // Combat and Trade
+    void proposeDuel(uint64_t targetGuid);
+    void initiateTrade(uint64_t targetGuid);
+    void stopCasting();
+
     // ---- Phase 1: Name queries ----
     void queryPlayerName(uint64_t guid);
     void queryCreatureInfo(uint32_t entry, uint64_t guid);