Function get_live_spectrum 

pub async fn get_live_spectrum(
    audio_service: State<'_, Mutex<AudioService>>,
) -> Result<SpectrumSnapshotDto, String>

Returns a single, immediate spectrum snapshot.

This command is useful for first paint / fallback reads before the push stream starts delivering live-spectrum events.

FFT analysis is offloaded to a blocking task so the async command handler never stalls the Tauri runtime thread.