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/ui/game_screen.cpp

@@ -124,7 +124,7 @@ void GameScreen::render(game::GameHandler& gameHandler) {
     renderVendorWindow(gameHandler);
     renderTrainerWindow(gameHandler);
     renderTaxiWindow(gameHandler);
-    renderQuestMarkers(gameHandler);  // 2D markers (3D M2 files not in MPQ)
+    // renderQuestMarkers(gameHandler);  // Disabled - using 3D billboard markers now
     renderMinimapMarkers(gameHandler);
     renderDeathScreen(gameHandler);
     renderResurrectDialog(gameHandler);