feat: add fcron command fallback

[volcano303]

What this changes

Adds support for systems using fcron/fcrontab instead of cronie crontab.

This introduces a shared cron command helper that:

• Uses crontab when available
• Falls back to fcrontab for local cron access
• Uses the same fallback behavior for remote SSH cron reads and writes

Closes #4.

How I tested this

Ran the full test suite:

.venv/bin/pytest -q

Result: 45 passed.

Checklist

• I have read CONTRIBUTING.md and this PR follows the guidelines
• A human has reviewed the entire diff of this PR, every line of code
• A human understands the changes and can explain why this approach is correct
• This PR doesn't have AI-generated boilerplate or co-author lines
• This PR was authored and submitted by an AI agent without human review

[antoniorodr]

Hello @volcano303!

Thanks for this PR. I do not use fcron, so I can't really test it thoroughly.

Have you installed to test it out? If so, can you share the steps you took to install it? I want to make sure that I can test it out before merging it.

And some edge cases here:

What if the machine/server is shared by multiple users, where one user uses fcron and another user uses crontab? Will there be any conflicts/issues? Will then be better to left the choice to the user if both fcron and crontab are installed?

I have not reviewed the PR yet, but I will do so as soon as I can. It has been a busy week for me, and the PR is "quite big", so I want to make sure I can give it the attention it deserves.

Thanks again!