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.

src/game/game_handler.cpp

@@ -2219,6 +2219,63 @@ void GameHandler::requestRaidInfo() {
     LOG_INFO("Requested raid info");
 }
 
+void GameHandler::proposeDuel(uint64_t targetGuid) {
+    if (state != WorldState::IN_WORLD || !socket) {
+        LOG_WARNING("Cannot propose duel: not in world or not connected");
+        return;
+    }
+
+    if (targetGuid == 0) {
+        addSystemChatMessage("You must target a player to challenge to a duel.");
+        return;
+    }
+
+    auto packet = DuelProposedPacket::build(targetGuid);
+    socket->send(packet);
+    addSystemChatMessage("You have challenged your target to a duel.");
+    LOG_INFO("Proposed duel to target: 0x", std::hex, targetGuid, std::dec);
+}
+
+void GameHandler::initiateTrade(uint64_t targetGuid) {
+    if (state != WorldState::IN_WORLD || !socket) {
+        LOG_WARNING("Cannot initiate trade: not in world or not connected");
+        return;
+    }
+
+    if (targetGuid == 0) {
+        addSystemChatMessage("You must target a player to trade with.");
+        return;
+    }
+
+    auto packet = InitiateTradePacket::build(targetGuid);
+    socket->send(packet);
+    addSystemChatMessage("Requesting trade with target.");
+    LOG_INFO("Initiated trade with target: 0x", std::hex, targetGuid, std::dec);
+}
+
+void GameHandler::stopCasting() {
+    if (state != WorldState::IN_WORLD || !socket) {
+        LOG_WARNING("Cannot stop casting: not in world or not connected");
+        return;
+    }
+
+    if (!casting) {
+        return; // Not casting anything
+    }
+
+    // Send cancel cast packet with current spell ID
+    auto packet = CancelCastPacket::build(currentCastSpellId);
+    socket->send(packet);
+
+    // Reset casting state
+    casting = false;
+    currentCastSpellId = 0;
+    castTimeRemaining = 0.0f;
+    castTimeTotal = 0.0f;
+
+    LOG_INFO("Cancelled spell cast");
+}
+
 void GameHandler::releaseSpirit() {
     if (!playerDead_) return;
     if (socket && state == WorldState::IN_WORLD) {