feat(kanban): add "Archive all" action to auto-complete columns

[reniko]

What

Adds an "Archive all" button to the header of Kanban columns whose status
has autoComplete: true. Clicking the button opens a confirmation dialog;
on confirm, all cards in that column are archived via the existing
archiveItem server action.

Why

Cards in auto-complete columns are effectively "done" and tend to
accumulate over time. Until now, users had to archive each card
individually — there was no bulk action.

How

• KanbanColumn renders an archive icon-button in the column header,
  conditional on the column's status having autoComplete: true and an
  onArchiveAll handler being provided (i.e. the user has edit permission).
• The button is disabled when the column is empty or while an archive
  pass is in flight.
• Clicking opens a ConfirmModal (destructive variant) with the affected
  column name and item count.
• Kanban.handleArchiveAll iterates the column's items sequentially,
  calling the existing archiveItem server action for each. The latest
  returned checklist state is propagated via onUpdate, and a final
  refreshChecklist() ensures consistency.
• On partial failure, the toast reports how many items were archived
  before the error; on full failure (zero archived), a plain failure
  toast is shown instead.

No new archive mechanism, no header refactor, no changes to the archive
data model.

Why sequential, not Promise.all

archiveItem writes to the file-based storage; parallel writes against
the same list file could race. Sequential is the safer default until
the storage layer is shown to handle concurrent writes for a single list.

Testing

• Button hidden in non-auto-complete columns
• Button visible in auto-complete columns (when user has edit permission)
• Button disabled when column is empty
• Button disabled during archive pass
• Single card: confirm → archived, appears in ArchivedItemsModal
• Many cards: confirm → all archived, no UI hang
• Cancel confirm → no-op
• de locale renders correctly

Translations

• en and de written by hand.
• Other languages: machine-translated, please review.
• klingon and pirate: in the spirit of the existing strings.

Out of scope

• Refactoring the column header layout
• Changes to the archive storage model
• Optimistic UI / progress indicator for very large columns