Mounts the aktivitaeten page with mocks for the notification SSE
singleton (init/destroy/markRead/markAllRead) and $app/state. Verifies
heading renders, error state renders main element, empty state renders
main, and a non-default filter renders without crashing.
4 tests covering the orchestration entry path.
Refs #496.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>