Function library_set_settings 

pub async fn library_set_settings(
    db: State<'_, DatabaseWrapper>,
    settings: LibrarySettings,
) -> Result<LibrarySettings, String>

Replace the library preferences: apply them to every subsequent query and persist them.

Returns the sanitised value actually applied, so the picker shows what was stored rather than what it sent.

TRACES: UR-076 | DR-209