Hide Duo panel when Duo availability is never on

What does this MR do and why?

When an administrator turns GitLab Duo off for the entire instance (duo_availability=never_on), the Duo panel still appeared in the sidebar for some users, showing either trial offer for admins, "Duo is disabled" empty state for other members. This change hides the panel completely in that case, so turning Duo off really removes it from the UI.

References

• Issue: #604800 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. As an instance administrator, apply:

   curl --request PUT --header "PRIVATE-TOKEN: <token>" \
        --data "duo_features_enabled=false&lock_duo_features_enabled=true&duo_availability=never_on" \
        "https://gitlab.example.com/api/v4/application/settings"

2. Sign in as admin member and ensure the Duo panel does not appear and can not be opened.

3. Sign in as developer member and ensure the Duo panel does not appear and can not be opened.

MR acceptance checklist

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