completeTask returns an undo closure that restores status + sort slot and
trashes the occurrence a repeat spawned. One global snackbar (5s,
replace-on-new) mounted in the root layout; every checkbox and Clarify's
save-as-done go through completeWithUndo. bun tests for the store.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YE5FmDZmFnWxH6RXCZLcjj
