x-ui.sh: install nftables alongside fail2ban in install_iplimit

On fresh Debian 12+, Ubuntu 24+ and recent RHEL-family minimal images
the fail2ban package ships with `banaction = nftables-multiport` as
the default in /etc/fail2ban/jail.conf but does not pull in the
`nftables` package as a dependency. The first SSH brute-force attempt
hits the default sshd jail and fail2ban logs

    stderr: /bin/sh: 1: nft: not found
    returned 127 -- HINT on 127: "Command not found"

repeatedly, which users mistake for a 3x-ui regression (see the
discussion on #4083). The 3x-ipl jail itself is unaffected — it uses
an iptables-based action configured in create_iplimit_jails — so this
is only stray noise, but noisy enough to look like a real failure on
first install.

Add `nftables` to the package list in every branch of install_iplimit
so new installs end up with a working default sshd jail out of the
box. Existing installs where `nftables` is already present are a
no-op.

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