feat: SongSelect panel — NAS song list with download/play in Menu.unity

- Add DownloadManager, SongLibrary, SongSelectManager, SongDetailPanel scripts
- Rebuild SongSelect panel inside Menu.unity using VRBeatsKit style:
  left scroll list (ALL/OWNED tabs) + right detail panel (diff buttons, Download/Delete/Play)
- SceneBuilder replaced: only ③ Menu — Rebuild SongSelect Panel remains
- All UI text in English

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ProjectSettings/EditorBuildSettings.asset

@@ -11,6 +11,9 @@ EditorBuildSettings:
   - enabled: 1
     path: Assets/VRBeatsKit/Scenes/BoxingStyle.unity
     guid: 63d51c8fe9633e54bb03cc51d243a0cd
+  - enabled: 1
+    path: Assets/Scenes/SongCreator.unity
+    guid: 09ef50fbbdfb41e4784b699dcffd4c2b
   - enabled: 1
     path: Assets/VRBeatsKit/Scenes/SaberStyle.unity
     guid: bb4ee84e66cce254e9cbf1fdfc136c08
@@ -19,3 +22,4 @@ EditorBuildSettings:
       type: 2}
     com.unity.xr.management.loader_settings: {fileID: 11400000, guid: 34230ee31c1362b49b1574bacd70986e,
       type: 2}
+  m_UseUCBPForAssetBundles: 0