(DevOps Pipelines) Hide Stage until edit mode is open

### Description

Would it be possible to hide the "Add stage" and only show it when editing the Pipeline?

For this:

an edit mode (by clicking on the current "Edit Pipeline Name")
hide the "Stage run" on the last stage (actually disabled)
hide the last ">"
hide "Add stage"
Once editing mode is closed, readjust for each stages the width or center it or I don't know 😅

Original post https://github.com/FlowFuse/flowfuse/issues/2902#issuecomment-1890996103

### Epic/Story

#2902

### Have you provided an initial effort estimate for this issue?

I am not a FlowFuse team member