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.

CMakeLists.txt

@@ -146,6 +146,7 @@ set(WOWEE_SOURCES
     src/rendering/character_preview.cpp
     src/rendering/wmo_renderer.cpp
     src/rendering/m2_renderer.cpp
+    src/rendering/quest_marker_renderer.cpp
     src/rendering/minimap.cpp
     src/rendering/world_map.cpp
     src/rendering/swim_effects.cpp