fix(chat): prevent AFK/DND auto-reply whisper spam loop

Auto-reply was sent on every incoming whisper with no dedup, causing
infinite loops when both players had auto-reply enabled. Now tracks
which senders have been replied to and only sends one auto-reply per
sender per AFK/DND session.

include/game/chat_handler.hpp

@@ -8,6 +8,7 @@
 #include <functional>
 #include <string>
 #include <unordered_map>
+#include <unordered_set>
 #include <vector>
 
 namespace wowee {
@@ -67,6 +68,10 @@ private:
     std::deque<MessageChatData> chatHistory_;
     size_t maxChatHistory_ = 100;
     std::vector<std::string> joinedChannels_;
+
+    // Track senders we've already auto-replied to (AFK/DND) this session
+    // to prevent infinite reply loops. Cleared when AFK/DND is toggled off.
+    std::unordered_set<std::string> afkAutoRepliedSenders_;
 };
 
 } // namespace game