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/zone_manifest.cpp

@@ -44,6 +44,17 @@ bool ZoneManifest::save(const std::string& path) const {
     if (hasCreatures) files["creatures"] = "creatures.json";
     j["files"] = files;
 
+    // Audio configuration
+    if (!musicTrack.empty() || !ambienceDay.empty()) {
+        nlohmann::json audio;
+        if (!musicTrack.empty()) audio["music"] = musicTrack;
+        if (!ambienceDay.empty()) audio["ambienceDay"] = ambienceDay;
+        if (!ambienceNight.empty()) audio["ambienceNight"] = ambienceNight;
+        audio["musicVolume"] = musicVolume;
+        audio["ambienceVolume"] = ambienceVolume;
+        j["audio"] = audio;
+    }
+
     std::ofstream f(path);
     if (!f) { LOG_ERROR("Failed to write zone manifest: ", path); return false; }
     f << j.dump(2) << "\n";
@@ -76,6 +87,16 @@ bool ZoneManifest::load(const std::string& path) {
             }
         }
 
+        // Audio configuration
+        if (j.contains("audio")) {
+            const auto& a = j["audio"];
+            musicTrack = a.value("music", "");
+            ambienceDay = a.value("ambienceDay", "");
+            ambienceNight = a.value("ambienceNight", "");
+            musicVolume = a.value("musicVolume", 0.7f);
+            ambienceVolume = a.value("ambienceVolume", 0.5f);
+        }
+
         return !mapName.empty();
     } catch (const std::exception& e) {
         LOG_ERROR("Failed to parse zone manifest: ", e.what());