Function print_folder_index_set 

pub fn print_folder_index_set(
    store: &EmailStore,
    config: &Config,
) -> Result<Vec<(String, String)>>

Generates per-folder index pages (date / subject / author / thread) for each subdirectory created by folder_by_date or msgsperfolder.

Returns (relative_path, html) pairs — the caller writes them to disk.