Audit Activity Sign-In Refusal Polish

Found during the Task 8271 UI check (Audit › Activity, operator view, desktop 1600px, local stack). 1. The refusal sentence shows the raw reason code ("Sign-in refused (unknown_email) via password"). Render the reason in words, e.g. "Sign-in refused: no account for this address (password)". 2. The audit event detail page shows no Account field for a row served no org. For the operator, show "Unattributed" there as the list's org slot does. 3. The floating "Get started" checklist covers the page's top-right toolbar area (the date filters' row edge) at 1600px. Dock it so it does not cover page controls, or collapse it by default on Audit. Out of scope: the search and facet behaviour Task 8271 shipped; the sign-in fold. Done means: each item is visible on the live stack in a screenshot; web tests cover 1 and 2; `./dc check quick` is green.

Refusal reasons in words, Unattributed Account on event detail, Get started dock railed on Audit. Quick gate green, 600 web tests pass, visual check done (all 3 visible at 1600px). PR #6438 queued in lane. Follow-up draft #8388 (fold org). Memory: none applied.