Improve the UX around Backgrounded Mirror Creation

### Is your feature request related to a problem?

Currently the pending flash doesn't go away until the user dismisses it or refreshes and there is no notice provided when a backgrounded mirror creation completes. Due to the chunked pushes, the repo is populated from old to new and users could potentially clone down an incomplete mirror. The in-progress mirror also appears in the mirror list and has options to rename or delete it which could break things.

### Describe the solution you'd like

The mirror creation UX should be enhanced to notify the user when a backgrounded mirror creation completes and a mirror that is still being created should not be able to be renamed or deleted with some indication to the user as to why.

### Describe alternatives you've considered

_No response_

### Additional context

Issues introduced and called out as part of https://github.com/github-community-projects/private-mirrors/pull/475

Start by tracing the backgrounded mirror creation flow and the mirror list actions for an in-progress mirror. Check how the pending flash is shown and cleared, then verify that completion is announced, incomplete mirrors cannot be cloned, and rename/delete are unavailable with an explanation while creation is running.

Written by the indexing model from the issue text.