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.hpp

@@ -150,6 +150,7 @@ private:
     float autoSaveInterval_ = 300.0f;
     bool autoSaveEnabled_ = true;
     bool showQuitConfirm_ = false;
+    ZoneManifest zoneManifest_;
 
     // Recent zones
     struct RecentZone { std::string mapName; int tileX; int tileY; };
@@ -162,6 +163,7 @@ public:
     void showToast(const std::string& msg, float duration = 3.0f);
     const std::vector<Toast>& getToasts() const { return toasts_; }
     const std::vector<RecentZone>& getRecentZones() const { return recentZones_; }
+    ZoneManifest& getZoneManifest() { return zoneManifest_; }
     bool isAutoSaveEnabled() const { return autoSaveEnabled_; }
     void setAutoSaveEnabled(bool v) { autoSaveEnabled_ = v; }
     float getAutoSaveInterval() const { return autoSaveInterval_; }