JSON Chart Structure Example:
{
  "id": "unique-id",
  "title": "Song Name",
  "artist": "Artist",
  "mp3": "https://example.com/song.mp3"  // or "data:audio/mpeg;base64,...",
  "offset": 0,
  "difficulty": "Normal",
  "notes": [
    { "time": 1.54, "lane": 1 },
    { "time": 2.13, "lane": 2 },
    { "time": 2.47, "lane": 3 },
    { "time": 2.80, "lane": 4 }
  ]
}