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/templates/audit/_rows.html

@@ -4,8 +4,8 @@
       {% for item in events %}
       {% set e = item.event %}
       <tr>
-        <td style="white-space:nowrap;" title="{{ e.timestamp.strftime('%Y-%m-%d %H:%M:%S UTC') }}">{{ e.timestamp|ago }}</td>
-        <td style="font-size:0.85rem;">{{ e.username }}</td>
+        <td class="nowrap">{{ e.timestamp|when }}</td>
+        <td class="nowrap">{{ e.username }}</td>
         <td title="{{ e.action }}">{{ item.action or e.action }}</td>
         <td>
           {% if item.entity %}
@@ -14,11 +14,9 @@
           {% if item.entity.gone %}<span class="field-hint">(deleted)</span>{% endif %}
           {% elif e.entity_type %}<span class="field-hint">{{ e.entity_type|replace('_', ' ') }}</span>{% endif %}
         </td>
-        <td style="font-size:0.8rem;color:var(--text-muted);max-width:340px;">
+        <td class="audit-facts">
           {% for f in item.facts %}
-          <span style="color:var(--text-dim);">{{ f.label }}</span>
-          {% if f.url %}<a href="{{ f.url }}">{{ f.text }}</a>{% else %}{{ f.text }}{% endif %}
-          {% if not loop.last %} &nbsp;·&nbsp; {% endif %}
+          <div><span class="tone-dim">{{ f.label }}</span> {% if f.url %}<a href="{{ f.url }}">{{ f.text }}</a>{% else %}{{ f.text }}{% endif %}</div>
           {% endfor %}
         </td>
       </tr>