Implement 3D quest markers using M2 models

Loads and renders actual quest marker M2 models from
World\Generic\PassiveDoodads\Quest\ as floating 3D objects above NPCs
based on quest status, replacing 2D ImGui text markers.

Features:
- Loads QuestExclamation.m2 (yellow !) for available quests
- Loads QuestQuestionMark.m2 (silver ?) for completable quests
- Updates marker positions dynamically as NPCs move
- Automatically spawns/despawns markers based on quest status changes
- Positions markers above NPC heads using render bounds

Quest markers are now proper 3D assets consistent with WoW 3.3.5a client.

include/core/application.hpp

@@ -203,6 +203,13 @@ private:
     };
     std::vector<PendingGameObjectSpawn> pendingGameObjectSpawns_;
     void processGameObjectSpawnQueue();
+
+    // Quest marker 3D models (billboarded above NPCs)
+    uint32_t questExclamationModelId_ = 0;
+    uint32_t questQuestionMarkModelId_ = 0;
+    std::unordered_map<uint64_t, uint32_t> questMarkerInstances_;  // npcGuid → marker instanceId
+    void loadQuestMarkerModels();
+    void updateQuestMarkers();
 };
 
 } // namespace core