feat(editor): zone audio configuration panel

- Zone manifest gains audio fields: musicTrack, ambienceDay,
  ambienceNight, musicVolume, ambienceVolume
- Serialized to/from zone.json under "audio" key
- Info panel: collapsable "Zone Audio" section with text inputs for
  music/ambience paths and volume sliders
- Preset selector: Elwynn Forest, Durotar, Darkshore, Dungeon, None
- ZoneManifest stored persistently on EditorApp so audio settings
  survive between exports (was recreated each save)
- Custom zones can now specify their own background music and ambient
  soundscapes via zone.json

tools/editor/editor_app.cpp

@@ -997,7 +997,7 @@ void EditorApp::exportZone(const std::string& outputDir) {
 
     // Write zone manifest (for client loading)
     // Scan output directory for all exported tiles (includes adjacent tiles)
-    ZoneManifest manifest;
+    ZoneManifest& manifest = zoneManifest_;
     manifest.mapName = loadedMap_;
     manifest.displayName = loadedMap_;
     manifest.tiles.push_back({loadedTileX_, loadedTileY_});