restore() fails when quarantine is on another filesystem

[Muawiya-contact]

quarantine() handles the cross-filesystem case:

if std::fs::rename(file, &dest).is_err() {
    std::fs::copy(file, &dest)...;
    std::fs::remove_file(file)...;
}

restore() does not — it's fs::rename and nothing else.

So the exact case the move handles is the case the undo fails on. The app puts quarantine in the app-local-data dir, so any file scanned from another mount (a second drive, /home on its own partition) hits this: quarantining works, restoring returns EXDEV.

Fix is to pull the rename-then-copy-fallback out into a shared helper and use it in both directions.

crates/diskern-core/src/actions.rs:60