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.

src/rendering/renderer.cpp

@@ -18,6 +18,7 @@
 #include "rendering/wmo_renderer.hpp"
 #include "rendering/m2_renderer.hpp"
 #include "rendering/minimap.hpp"
+#include "rendering/quest_marker_renderer.hpp"
 #include "rendering/shader.hpp"
 #include "pipeline/m2_loader.hpp"
 #include <algorithm>
@@ -330,6 +331,9 @@ bool Renderer::initialize(core::Window* win) {
         minimap.reset();
     }
 
+    // Create quest marker renderer (initialized later with AssetManager)
+    questMarkerRenderer = std::make_unique<QuestMarkerRenderer>();
+
     // Create M2 renderer (for doodads)
     m2Renderer = std::make_unique<M2Renderer>();
     // Note: M2 renderer needs asset manager, will be initialized when terrain loads
@@ -1871,6 +1875,11 @@ void Renderer::renderWorld(game::World* world) {
         waterRenderer->render(*camera, time);
     }
 
+    // Render quest markers (billboards above NPCs)
+    if (questMarkerRenderer && camera) {
+        questMarkerRenderer->render(*camera);
+    }
+
     // Full-screen underwater tint so WMO/M2/characters also feel submerged.
     if (false && underwater && underwaterOverlayShader && underwaterOverlayVAO) {
         glDisable(GL_DEPTH_TEST);
@@ -2224,6 +2233,9 @@ bool Renderer::loadTestTerrain(pipeline::AssetManager* assetManager, const std::
         if (movementSoundManager) {
             movementSoundManager->initialize(assetManager);
         }
+        if (questMarkerRenderer) {
+            questMarkerRenderer->initialize(assetManager);
+        }
         cachedAssetManager = assetManager;
     }
 
@@ -2323,6 +2335,9 @@ bool Renderer::loadTerrainArea(const std::string& mapName, int centerX, int cent
     if (movementSoundManager && cachedAssetManager) {
         movementSoundManager->initialize(cachedAssetManager);
     }
+    if (questMarkerRenderer && cachedAssetManager) {
+        questMarkerRenderer->initialize(cachedAssetManager);
+    }
 
     // Wire ambient sound manager to terrain manager for emitter registration
     if (terrainManager && ambientSoundManager) {