i15-1: Do not allow queuing items that are not mounted to the table

[DominicOram]

Following #159 items that are not queue-able are highlighted but the UI still lets you queue them. As per the discussion on https://diamondlightsource.slack.com/archives/C08FER0BZN0/p1785869701856729 we should not allow this and give a nice message to the user as to why

[jacob-williamson]

If we queued everything at the same time instead of doing one request at a time, the queue would reject the whole operation and nothing would get added