If the remote is ahead of our local mirror (non-fast-forward), the old
code used --force which silently destroyed remote history. Fix:
- Drop --force from the git push invocation.
- On non-ff exit, detect the condition and return Ok(()) instead of
bailing (intentional no-op; leaving remote history intact is correct).
- Raise a persistent dashboard warning banner via crate::warnings so the
operator sees it in the UI rather than having to grep the journal.
- Clear the banner on the next successful push.
Closes#2380.