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>

steward/audit/service.py

@@ -92,6 +92,7 @@ ACTIONS = {
     "auth.login": "signed in",
     "auth.login_failed": "failed to sign in",
     "auth.setup": "set up Steward",
+    "account.time_zone": "set their time zone",
     "host.created": "added host",
     "host.renamed": "renamed host",
     "host.deleted": "deleted host",
@@ -143,7 +144,7 @@ ACTIONS = {
 }
 
 # The filter's choices: an area, matched as a prefix of the action code.
-AREAS = (("auth.", "Sign-ins"), ("host.", "Hosts"), ("monitor.", "Monitors"),
+AREAS = (("auth.", "Sign-ins"), ("account.", "Accounts"), ("host.", "Hosts"), ("monitor.", "Monitors"),
          ("alert_rule.", "Alert rules"), ("maintenance_window.", "Maintenance"),
          ("ansible.run", "Playbook runs"), ("ansible.", "Ansible (all)"),
          ("settings.", "Settings"), ("plugin.", "Plugins"), ("api_token.", "AI access"),