Add a single-line countdown timer

[utk2103]

Add a countdown timer that updates on a single line in the terminal.

Why

Tiny script, two new ideas: time.sleep for pacing, and \r + flush=True to overwrite a line instead of spamming 300 lines of output. Everyone's first timer prints a new line every second — this one shouldn't.

Suggested file

scripts/countdown_timer.py

What it should do

• Usage: python scripts/countdown_timer.py 90 or python scripts/countdown_timer.py 1:30
• Display MM:SS counting down in place, then print a finish message and beep (print("\a")).

Acceptance criteria

• Single-line update using \r and end="", flush=True — not one line per tick.
• Accepts plain seconds and MM:SS format.
• Ctrl+C prints the remaining time and exits cleanly, no traceback.
• Standard library only (time, argparse).

Inspiration

Countdown from Awesome-Python-Scripts.

Good first issue — comment below to get assigned.