Add countdown timer mode

[Priyanshu20511]

Summary

This pull request adds a countdown timer mode to tty-clock.

Features

• Added a new -M option to start a countdown timer.
• Supports durations using s, m, and h suffixes (e.g. 10s, 5m, 1h).
• Displays the remaining time using the existing tty-clock interface.
• Stops the countdown at 00:00:00 when completed.
• Updated the help message to document the new option.

Testing

Tested with:

• tty-clock -M 10s
• tty-clock -M 5m
• tty-clock -M 1h
• Normal clock mode
• Seconds display (-s)
• Rebound mode (-r)
• Reset ('o' or 'O')
• Pause ('p' or 'P')
  The existing clock functionality remains unchanged.