fix(soup): use sortTs for email thread ordering and display

[evanhutnik]

Summary

Email threads in soup were displaying updatedAt as the timestamp and being client-side sorted by updatedAt/createdAt/viewedAt, which does not match the backend actual sort order (effective_ts / sortTs). This caused emails to appear in the wrong order and show incorrect timestamps.

• Add sortTs to EntityBase and map it from the soup API response for email threads
• Use sortTs as the preferred timestamp in EntityTimestamp and SoupViewList for emails
• Update client-side sort comparators (sortByUpdatedAt, sortByCreatedAt, sortByViewedAt) to prefer sortTs when present
• Non-email entities are unaffected — they do not have sortTs and retain existing behavior

[github-actions]

Preview: https://evan-fix-soup-ordering-t4y3s9.preview.macro.com/app (0a4d639)