Implement WoW-style 3D billboard quest markers

Replace 2D ImGui text markers with proper 3D billboard sprites using BLP textures.

Features:
- Billboard rendering using Interface\GossipFrame\ BLP textures (yellow !, yellow ?, grey ?)
- WoW-style visual effects: bob animation, distance-based scaling, glow pass, distance fade
- Proper NPC height positioning with bounding box detection
- Camera-facing quads with depth testing but no depth write
- Shader-based alpha modulation for glow and fade effects

Technical changes:
- Created QuestMarkerRenderer class with billboard sprite system
- Integrated into Renderer initialization for both online and offline terrain loading
- Rewrote updateQuestMarkers() to use billboard system instead of M2 models
- Disabled old 2D ImGui renderQuestMarkers() in game_screen.cpp
- Added debug logging for initialization and marker tracking

Quest markers now render with proper WoW visual fidelity.

include/core/application.hpp

@@ -204,12 +204,9 @@ 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();
+    // Quest marker billboard sprites (above NPCs)
+    void loadQuestMarkerModels();  // Now loads BLP textures
+    void updateQuestMarkers();     // Updates billboard positions
 };
 
 } // namespace core