Copy text in share launch link modal

[leafty]

minor detail, but I think the customize section should be different when no env var can be set (see screenshot). I think maybe removing the "Select.." text in this case would work. Also, wording should be fixed to "Select the parameters to customize of your session launch link."

https://swiss-data-science.slack.com/archives/C03E66DHUG3/p1754550708900019