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>

steward/templates/settings/general.html

@@ -2,10 +2,10 @@
 {% extends "base.html" %}
 {% from "_macros.html" import crumbs %}
 {% block title %}Settings — General — Steward{% endblock %}
+{% from "settings/_tabs.html" import settings_tabs %}
+{% block section_tabs %}{{ settings_tabs("general") }}{% endblock %}
 {% block breadcrumb %}{{ crumbs([("Settings", "/settings/"), ("General", "")]) }}{% endblock %}
 {% block content %}
-{% set active_tab = "general" %}
-{% include "settings/_tabs.html" %}
 
 <form method="post" action="/settings/general/">{{ csrf_field() }}
 <div class="card" style="max-width:640px;">
@@ -72,7 +72,7 @@
     <label>Monitor poll interval <span style="color:var(--text-muted);font-size:0.8rem;">(seconds)</span></label>
     <input type="number" name="monitors.poll_interval_seconds" min="10"
            value="{{ settings['monitors.poll_interval_seconds'] }}" style="max-width:120px;">
-    <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.25rem;">Requires restart to take effect.</div>
+    <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.25rem;">How often each monitor is checked, unless it sets its own. Takes effect on the next check.</div>
   </div>
 
   <div class="form-group">