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/list.html

@@ -1,10 +1,12 @@
+{# audit/list.html — who changed what, newest first, each moment exact in the
+   reader's own zone (milestone 490): a record is read for when, not how long ago. #}
 {% extends "base.html" %}
 {% from "_macros.html" import page_header, empty_state %}
 {% block title %}Audit log — Steward{% endblock %}
 {% block content %}
 {% call page_header("Audit log", "Who changed what, newest first.") %}
-  <form method="get" style="display:flex;gap:0.5rem;align-items:center;">
-    <select name="action" aria-label="Show only" style="font-size:0.85rem;" onchange="this.form.submit()">
+  <form class="filter-bar" method="get" action="/audit/">
+    <select name="action" aria-label="Show only" onchange="this.form.submit()">
       <option value="">Everything</option>
       {% for prefix, label in areas %}
       <option value="{{ prefix }}" {% if action_filter == prefix %}selected{% endif %}>{{ label }}</option>
@@ -19,7 +21,7 @@
   <table class="table">
     <thead>
       <tr>
-        <th>When</th>
+        <th>When ({{ tz_label() }})</th>
         <th>User</th>
         <th>Did</th>
         <th>What</th>