Handle edge cases and cleanup

[Pupler]

Tasks:

• Remove dead null-check (group.Key != null, f.Name?[...])
  once confirmed Name can never be null at that point
• Decide what to do with files that have no dash (currently
  skipped with console log — should they go to an "unsorted"
  folder instead?)
• Handle case-sensitivity of prefixes (e.g. "photo-1" vs "Photo-2"
  — same group or different?)
• Add basic error handling around File.Move (permissions,
  file in use, etc.)