feat(settings): Settings brought into line with the rest of Steward (milestone 493)

The last page of #5214's walk. Settings still described Steward as it was
before monitors were unified and SNMP was rebuilt, drew its tabs below the
breadcrumb, and showed the report schedule in UTC.

- Tabs (#5377): settings/_tabs.html is a settings_tabs() macro called in
  each page's section_tabs block, above the trail, as Ansible's are. The
  breadcrumb guard also fails any page that {% include %}s a tab strip.
- Report time (#5378): reports.schedule_tz keeps the zone of whoever saved
  the schedule; check_and_send compares in it (report_due), and the form shows
  the day and hour on the reader's clock (schedule_in) under their zone's
  name. "Last sent" uses |when.
- Wording (#5379): the report contents list what the report holds (monitors,
  not ping-enabled hosts); "Ping latency" is "Response time" and says what it
  grades; plugin detail says enabling loads now and disabling needs a
  restart; the monitor interval says it applies on the next check (the
  scheduler reads it every pass); the dead list-config branch is gone.
- Traefik (#5380): the 5xx and 4xx cutoffs are settings
  (traefik.threshold.error_5xx_pct 0.1, error_4xx_pct 1.0) in a "Traefik
  routers" group on Thresholds while the plugin runs, read by router_state
  through error_cutoffs(). The Traefik page has a Thresholds button for
  admins, to /settings/thresholds/#traefik.

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

plugins/traefik/templates/traefik/index.html

@@ -7,7 +7,12 @@
     <h1 class="page-title" style="margin-bottom:0;">Traefik Services</h1>
     <span style="color:var(--text-muted);font-size:0.85rem;">polling every {{ poll_interval }}s</span>
   </div>
-  {% include "_time_range.html" %}
+  <div style="display:flex;align-items:center;gap:0.75rem;flex-wrap:wrap;">
+    {% if session.user_role == 'admin' %}
+    <a href="/settings/thresholds/#traefik" class="btn btn-ghost btn-sm">Thresholds</a>
+    {% endif %}
+    {% include "_time_range.html" %}
+  </div>
 </div>
 
 <div id="traefik-rows"