Flatpak: Repair an Offline Mirror Handover

An internal Flatpak mirror handover mixed up two installation scopes. Read /root/case/brief.txt, repair the existing system remote, add the same name in root's per-user installation using its different mirror, and remove the retired system remote without disturbing the reference user remote. This lab uses real Flatpak and local empty OSTree repositories with no internet access. You are root, so --user means root's personal installation. Grading inspects the actual remote names, URLs and enabled state. It does not grade application installation, removal, or GUI operation.