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/account/index.html

@@ -0,0 +1,32 @@
+{# account/index.html — the signed-in user's own account (milestone 490): who
+   they are, and the time zone every page writes its times in. #}
+{% extends "base.html" %}
+{% from "_macros.html" import page_header %}
+{% block title %}Your account — Steward{% endblock %}
+{% block content %}
+{{ page_header("Your account") }}
+
+<p class="page-verdict">
+  Signed in as <strong>{{ user.username }}</strong> ({{ user.role.value }}) with {{ source }}
+  · times show in <strong>{{ zone_name }}</strong> ({{ zone_label }})
+</p>
+
+<form method="post" action="/account/" class="form-page">{{ csrf_field() }}
+  <section class="card form-section" aria-labelledby="acct-time">
+    <h2 id="acct-time" class="section-title">Time zone</h2>
+    {% if error %}<p class="field-error" role="alert">{{ error }}</p>{% endif %}
+    <div class="field">
+      <label class="field-label" for="acct-tz">Show times in</label>
+      <select class="input" id="acct-tz" name="time_zone">
+        {% for region, names in zones %}
+        {% if region %}<optgroup label="{{ region }}">{% endif %}
+        {% for z in names %}<option value="{{ z }}"{% if z == zone_name %} selected{% endif %}>{{ z.split('/', 1)[1]|replace('_', ' ') if region else z }}</option>{% endfor %}
+        {% if region %}</optgroup>{% endif %}
+        {% endfor %}
+      </select>
+      <span class="field-hint">Every page writes its times in this zone, on a 24-hour clock.</span>
+    </div>
+  </section>
+  <button type="submit" class="btn">Save</button>
+</form>
+{% endblock %}