feat: show area trigger messages as screen banners

SMSG_AREA_TRIGGER_MESSAGE events (dungeon enter messages, objective
triggers, etc.) were previously only appended to chat. Now they also
appear as animated slide-up toasts in the lower-center of the screen:
blue-bordered dark panel with light-blue text, 4.5s lifetime with
35ms slide-in/out animation. Up to 4 simultaneous toasts stack
vertically. Messages still go to chat as before.

include/ui/game_screen.hpp

@@ -119,6 +119,10 @@ private:
     // Zone entry toast: brief banner when entering a new zone
     struct ZoneToastEntry { std::string zoneName; float age = 0.0f; };
     std::vector<ZoneToastEntry> zoneToasts_;
+
+    struct AreaTriggerToast { std::string text; float age = 0.0f; };
+    std::vector<AreaTriggerToast> areaTriggerToasts_;
+    void renderAreaTriggerToasts(float deltaTime, game::GameHandler& gameHandler);
     std::string lastKnownZone_;
     static constexpr float kZoneToastLifetime = 3.0f;