fix(sync): improve shelf book synchronization and filename generation

    if not book or type(book) ~= "table" then return book end

    if book.metadata and type(book.metadata) == "table" then

        -- Title lives under metadata on the shelf endpoint

        book.title = book.title or book.metadata.title

        -- First author as scalar string

        if book.metadata.authors and type(book.metadata.authors) == "table"

           and #book.metadata.authors > 0 then

                ::book_continue::

            end

            -- Bidirectional sync: remove local files no longer on shelf

            -- Bidirectional sync: remove local files no longer on shelf.

            -- Uses DB lookup rather than filename pattern so that title-based

            -- filenames (and legacy BookID_* files that have a cache entry) are

            -- handled uniformly.  Files with no DB entry are left untouched.

            if self.delete_removed_shelf_books then

                for entry in lfs.dir(self.download_dir) do

                    local local_id = entry:match("^BookID_(%d+)%.[%a]+$")

                    if local_id then

                        local lid = tonumber(local_id)

                        if lid and not shelf_book_ids[lid] then

                    if entry ~= "." and entry ~= ".." then

                        local fp = self.download_dir .. "/" .. entry

                        if lfs.attributes(fp, "mode") == "file" then

                            local cached = self.db:getBookByFilePath(fp)

                            local lid = cached and cached.book_id

                            if lid and not shelf_book_ids[lid] then

                                UIManager:scheduleIn(0, function()

                                    if info_msg then

                                    info_msg.text = T(_("Removing: BookID %1"), lid)

                                        info_msg.text = T(_("Removing: %1"), entry)

                                        UIManager:forceRePaint()

                                    end

                                end)

                        end

                    end

                end

            end

        end)

        if ok_loop then

end

--[[--

Generate a stable filename for a shelf book download.

Generate a filename for a shelf book download.

Format: "BookID_{id}.{extension}" - avoids filesystem-unsafe characters

while keeping filenames predictable for the bidirectional deletion sweep.

Uses the book title as the filename, sanitized of filesystem-unsafe characters

and truncated so the full filename (including extension) is at most 150 chars.

Falls back to "BookID_{id}.{extension}" when no usable title is available.

@param book table  Book object from Booklore API

@return string

--]]

function BookloreSync:_generateFilename(book)

    local extension = (book.extension or "epub"):lower()

    return "BookID_" .. tostring(book.id) .. "." .. extension

    local ext_suffix = "." .. extension

    local title = book.title

    if title and title ~= "" then

        -- Strip characters that are illegal on common filesystems (FAT32, NTFS, ext4)

        local safe = title

            :gsub('[/\\:*?"<>|]', "")

            :gsub("%s+", " ")

            :gsub("^%s+", "")

            :gsub("%s+$", "")

        if safe ~= "" then

            local max_stem = 150 - #ext_suffix

            if #safe > max_stem then

                safe = safe:sub(1, max_stem):gsub("%s+$", "")

            end

            return safe .. ext_suffix

        end

    end

    -- Fallback when title is absent or becomes empty after sanitization

    return "BookID_" .. tostring(book.id) .. ext_suffix

end

--[[--

end

return BookloreSync