.backup-retention-preset-block carried a permanent border-bottom (+ padding),
but the Keep-* fields it separates the dropdown from are hidden for every
preset except Custom — so on Self-hosting/etc. it was an orphaned line with
empty space beneath it. Moved the divider + spacing onto an .is-custom state
that applyVisibility() toggles alongside the fields.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
41786edd36
