feat: add /stopmacro support and low durability warning for equipped items

- /stopmacro [conditions] halts remaining macro commands; supports all existing
  macro conditionals ([combat], [nocombat], [mod:shift], etc.) via the sentinel
  action trick on evaluateMacroConditionals
- macroStopped_ flag in GameScreen; executeMacroText resets and checks it after
  each command so /stopmacro mid-macro skips all subsequent lines
- Emit a "X is about to break!" UI error + system chat when an equipped item's
  durability drops below 20% via SMSG_UPDATE_OBJECT field delta; warning fires
  once per threshold crossing (prevDur >= maxDur/5, newDur < maxDur/5)

include/ui/game_screen.hpp

@@ -55,6 +55,9 @@ private:
     std::vector<std::string> chatSentHistory_;
     int chatHistoryIdx_ = -1;  // -1 = not browsing history
 
+    // Set to true by /stopmacro; checked in executeMacroText to halt remaining commands.
+    bool macroStopped_ = false;
+
     // Tab-completion state for slash commands
     std::string chatTabPrefix_;            // prefix captured on first Tab press
     std::vector<std::string> chatTabMatches_;  // matching command list