Empty state (default + term-specific), error banner, year groups
default sort, sender-group sort, undated/unknown-sender labels, total
count display. Mocks $app/navigation since the empty-state CTA calls
goto.
8 tests covering ~30 of DocumentList's branches.
Refs #496.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
