Moves the confirm-then-delete flow out of StoryReader and JourneyReader into
the single [id]/+page.svelte owner. Both reader components gain an optional
ondelete prop — the delete button calls ondelete?.() so the handler is opt-in
and never duplicated. Tests verify the prop is called on click.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>