feat: times in the reader's own zone; the audit log shows exact moments (milestone 490)

- users.time_zone (0034). An account without one takes the browser's zone
  on its first page (POST /account/time-zone/adopt, never over a choice made
  by hand); the session follows the account on every request.
- /account: who you are and the zone every page writes its times in,
  reached from your name in the sidebar.
- formatting.when: 'Today 06:58', 'Yesterday 23:41', 'Tue 6 Oct 14:12',
  '6 Oct 2025 14:12', 24-hour, in the reader's zone; |ago's hover is the
  exact local moment instead of UTC; tz_label() for column headers.
- Audit log: When (EDT) with exact times, one detail fact a line, on classes.
- tzdata in the image, and the airgapped smoke checks a zone resolves.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_breadcrumbs.py

@@ -18,6 +18,7 @@ TOP_LEVEL = {
     "steward/templates/ansible/index.html",
     "steward/templates/audit/list.html",
     "steward/templates/ai_access/index.html",
+    "steward/templates/account/index.html",
     "plugins/docker/templates/docker/index.html",
     "plugins/snmp/templates/snmp/index.html",
     "plugins/traefik/templates/traefik/index.html",