BUG: sanitizeFilename truncates by bytes, can produce invalid UTF-8

[andrinoff]

Describe the bug

main.go:2808-2815:

const maxFilenameLen = 255
if len(name) > maxFilenameLen {
    ext := filepath.Ext(name)
    ...
    name = name[:maxFilenameLen-len(ext)] + ext
}

The slice cuts at byte boundaries. If the filename is multi-byte UTF-8 (CJK, emoji, accented letters) the cut can land mid-rune, producing an invalid UTF-8 filename — which several filesystems then reject or display as ?.

Expected behavior

Trim by rune boundary using utf8.DecodeLastRuneInString so the last rune is whole.