feat: add /clear chat command and movement speed display in stats

- /clear slash command empties the chat history (was listed in
  autocomplete but never handled)
- Stats panel shows run/flight/swim speed as percentage of base only
  when non-default (e.g. mounted or speed-buffed), under a new
  Movement section

include/game/game_handler.hpp

@@ -283,6 +283,7 @@ public:
      * @return Vector of chat messages
      */
     const std::deque<MessageChatData>& getChatHistory() const { return chatHistory; }
+    void clearChatHistory() { chatHistory.clear(); }
 
     /**
      * Add a locally-generated chat message (e.g., emote feedback)