Add progress indicator for large directories

[MatthewMckee4]

Summary

When counting large directories, kount can appear to hang with no feedback. Add a progress indicator (spinner or progress bar) to stderr so the user knows work is happening.

Considerations

• Only show when stderr is a terminal (not when piped)
• Should not interfere with stdout output
• Use a lightweight crate like indicatif
• Show files counted so far and optionally a spinner

Acceptance criteria

• Progress indicator shown on stderr during counting
• Only displayed when stderr is a TTY
• Does not appear in piped/redirected output
• Cleared before final results are printed to stdout