Add Update as a conditional 6th option in the interactive menu

[xde013]

Currently, updating Mole requires knowing and typing mo update each time...
Now when a new version is detected (via the existing update_message cache), a numbered Update entry appears as option 6 in the mo interactive menu.

Changes:

• show_main_menu: renders option 6 only when MAIN_MENU_SHOW_UPDATE is true
• interactive_main_menu: sets MAIN_MENU_SHOW_UPDATE from the existing update_message cache, keeping it as the single source of truth
• Arrow key DOWN bound is capped at 5 or 6 depending on the flag
• ENTER case 6 and CHAR:6 shortcut both guard on MAIN_MENU_SHOW_UPDATE -> pressing 6 when no update is available is a no-op, consistent with the hidden option
• tests/completion.bats: adds update and update:Update to latest version assertions to the bash and zsh completion tests

[tw93]

@xde013 Thanks a lot for the contribution and for putting this together.

I didn’t merge #540 directly, but I applied the underlying fix on main and linked it as a follow-up to #540.

The main reason is that the PR included more than this specific menu behavior, and the update action could still be triggered through the U shortcut even when the entry was hidden. That behavior is now fixed on main, and I also added test coverage for both the hidden and visible cases.

So there’s no need to merge #540 for this specific change anymore, but I really appreciate the work here.