Add support for authenticated deletion of remote branches and folders.
The backend adds a batch delete command that queries the remote,
performs an atomic push --delete when possible with a fallback, and
prunes stale tracking refs; it accepts optional credentials. The
frontend and JS API integrate credential handling and prompt users when
authentication is required.
- New batch deletion command with optional username/password for auth.
- Uses ls-remote to scope deletions, tries --atomic then falls back.
- Frontend updates: credential dialog/action, pendingRemoteDelete state.
Christoph
0dac3e9f09