The previous version navigated to /briefwechsel with no params, which
renders the hero state — axe-core scanned the hero, not the new
ThumbnailRow / ConversationThumbnail / DistributionBar. This commit
seeds two persons + one document via the API in beforeAll, then
drives the URL with ?senderId=X&receiverId=Y so each of the
36 test runs (3 viewports × 2 themes × 2 assertions) actually scans
the intended DOM. Also asserts that conv-person-bar is visible first,
so a regression that drops the page back to hero fails explicitly
rather than silently passing an empty sweep.
Refs #305
Fixes @saraholt blocker 1 from PR review
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>