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/rendering/quest_marker_renderer.hpp

@@ -0,0 +1,71 @@
+#pragma once
+
+#include <glm/glm.hpp>
+#include <cstdint>
+#include <vector>
+#include <unordered_map>
+
+namespace wowee {
+namespace pipeline { class AssetManager; }
+namespace rendering {
+
+class Camera;
+
+/**
+ * Renders quest markers as billboarded sprites above NPCs
+ * Uses BLP textures from Interface\GossipFrame\
+ */
+class QuestMarkerRenderer {
+public:
+    QuestMarkerRenderer();
+    ~QuestMarkerRenderer();
+
+    bool initialize(pipeline::AssetManager* assetManager);
+    void shutdown();
+
+    /**
+     * Add or update a quest marker at a position
+     * @param guid NPC GUID
+     * @param position World position (NPC base position)
+     * @param markerType 0=available(!), 1=turnin(?), 2=incomplete(?)
+     * @param boundingHeight NPC bounding height (optional, default 2.0f)
+     */
+    void setMarker(uint64_t guid, const glm::vec3& position, int markerType, float boundingHeight = 2.0f);
+
+    /**
+     * Remove a quest marker
+     */
+    void removeMarker(uint64_t guid);
+
+    /**
+     * Clear all markers
+     */
+    void clear();
+
+    /**
+     * Render all quest markers (call after world rendering, before UI)
+     */
+    void render(const Camera& camera);
+
+private:
+    struct Marker {
+        glm::vec3 position;
+        int type; // 0=available, 1=turnin, 2=incomplete
+        float boundingHeight = 2.0f;
+    };
+
+    std::unordered_map<uint64_t, Marker> markers_;
+    
+    // OpenGL resources
+    uint32_t vao_ = 0;
+    uint32_t vbo_ = 0;
+    uint32_t shaderProgram_ = 0;
+    uint32_t textures_[3] = {0, 0, 0}; // available, turnin, incomplete
+
+    void createQuad();
+    void loadTextures(pipeline::AssetManager* assetManager);
+    void createShader();
+};
+
+} // namespace rendering
+} // namespace wowee

include/rendering/renderer.hpp

@@ -32,6 +32,7 @@ class CharacterRenderer;
 class WMORenderer;
 class M2Renderer;
 class Minimap;
+class QuestMarkerRenderer;
 class Shader;
 
 class Renderer {
@@ -105,6 +106,7 @@ public:
     WMORenderer* getWMORenderer() const { return wmoRenderer.get(); }
     M2Renderer* getM2Renderer() const { return m2Renderer.get(); }
     Minimap* getMinimap() const { return minimap.get(); }
+    QuestMarkerRenderer* getQuestMarkerRenderer() const { return questMarkerRenderer.get(); }
     const std::string& getCurrentZoneName() const { return currentZoneName; }
 
     // Third-person character follow
@@ -177,6 +179,7 @@ private:
     std::unique_ptr<WMORenderer> wmoRenderer;
     std::unique_ptr<M2Renderer> m2Renderer;
     std::unique_ptr<Minimap> minimap;
+    std::unique_ptr<QuestMarkerRenderer> questMarkerRenderer;
     std::unique_ptr<audio::MusicManager> musicManager;
     std::unique_ptr<audio::FootstepManager> footstepManager;
     std::unique_ptr<audio::ActivitySoundManager> activitySoundManager;