Show checkable lists of existing targets and remotes instead of
free-text comma-separated inputs. Selected items are joined with
commas on save, empty selection means "all".
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Replace `is Select.BLANK` / `is not Select.BLANK` with isinstance
checks, as the sentinel identity varies across Textual versions.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>