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

src/ui/game_screen.cpp

@@ -4669,6 +4669,12 @@ void GameScreen::sendChatMessage(game::GameHandler& gameHandler) {
                 return;
             }
 
+            if (cmdLower == "clear") {
+                gameHandler.clearChatHistory();
+                chatInputBuffer[0] = '\0';
+                return;
+            }
+
             // /invite command
             if (cmdLower == "invite" && spacePos != std::string::npos) {
                 std::string targetName = command.substr(spacePos + 1);