Update PMOMusic/Cargo.toml and version.txt from 0.3.59 to 0.3.60. Add explicit application/manifest+json detection for .webmanifest files in serve_embed.rs to compensate for mime_guess limitations, converting the MIME value to an owned String and updating the CONTENT_TYPE header reference accordingly.
- Drop `Path`/``State``` from unused Axum imports in config.rs and registry
- Mark `_position_sec` field as `#[allow(dead_code)]`` in PositionUpdateRequest and PlayerStateReport
- Remove unused macro rules (`add_action_arg!`, `add_action!``, `` add_var!)``
- Delete unused PlayerReport struct and related handler code
