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/thresholds.html

@@ -2,10 +2,10 @@
 {% extends "base.html" %}
 {% from "_macros.html" import crumbs %}
 {% block title %}Settings — Thresholds & Retention — Steward{% endblock %}
+{% from "settings/_tabs.html" import settings_tabs %}
+{% block section_tabs %}{{ settings_tabs("thresholds") }}{% endblock %}
 {% block breadcrumb %}{{ crumbs([("Settings", "/settings/"), ("Thresholds & retention", "")]) }}{% endblock %}
 {% block content %}
-{% set active_tab = "thresholds" %}
-{% include "settings/_tabs.html" %}
 
 {% macro pair(label, warn_field, warn_key, crit_field, crit_key, unit, hint, step="1") %}
 <div class="form-group" style="margin-bottom:1.1rem;">
@@ -40,7 +40,7 @@
   {{ pair("Disk usage",   "disk_warn", "thresholds.disk_warn", "disk_crit", "thresholds.disk_crit", "%", "Filesystem usage.") }}
   {{ pair("Load",         "load_warn", "thresholds.load_warn", "load_crit", "thresholds.load_crit", "% per core", "Load average normalized by core count (100% = one full core per core).") }}
   {{ pair("Temperature",  "temp_warn", "thresholds.temp_warn", "temp_crit", "thresholds.temp_crit", "°C", "Hottest reported sensor per host.") }}
-  {{ pair("Ping latency", "latency_warn", "ping.threshold.good_ms", "latency_crit", "ping.threshold.warn_ms", "ms", "Round-trip time. Shared with the Ping widget's good/warn colouring.") }}
+  {{ pair("Response time", "latency_warn", "ping.threshold.good_ms", "latency_crit", "ping.threshold.warn_ms", "ms", "How long a monitor's check takes to answer. Also grades Traefik routers' p95 and the UniFi WAN round trip.") }}
   {{ pair("Uptime / SLA", "uptime_warn", "thresholds.uptime_warn", "uptime_crit", "thresholds.uptime_crit", "%", "Lower is worse — amber below warn, red below crit.", step="0.1") }}
   {{ pair("TLS certificate expiry", "tls_warn", "thresholds.tls_warn_days", "tls_crit", "thresholds.tls_crit_days", "days left", "HTTPS monitors whose certificate expires within these many days are flagged. Lower is worse.") }}
   {{ pair("Clock drift", "clock_warn", "thresholds.clock_warn_s", "clock_crit", "thresholds.clock_crit_s", "seconds", "How far a host's clock may be from Steward's, either way, before its page and the Hosts list call it out.") }}
@@ -57,6 +57,29 @@
   </div>
 </div>
 
+{# Traefik's router cutoffs, while the plugin runs; its page links here (#5380). #}
+{% if traefik_on %}
+<div class="card" id="traefik" style="max-width:640px;margin-top:1.25rem;">
+  <h2 class="section-title" style="margin-bottom:0.5rem;">Traefik routers</h2>
+  <p style="font-size:0.82rem;color:var(--text-muted);margin-bottom:1.25rem;">
+    How much of a router's traffic, over the last 15 minutes, may end in an error before the
+    <a href="/plugins/traefik/">Traefik page</a> and the Routers widget list it.
+  </p>
+  <div class="form-group" style="margin-bottom:1.1rem;">
+    <label for="error_5xx_pct">Server errors (5xx) <span style="color:var(--text-muted);font-size:0.8rem;">(% of responses)</span></label>
+    <input type="number" id="error_5xx_pct" name="error_5xx_pct" min="0" max="100" step="0.01"
+           value="{{ settings['traefik.threshold.error_5xx_pct'] }}" style="max-width:110px;margin-top:0.25rem;">
+    <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.3rem;">Above this share, a router is failing (red).</div>
+  </div>
+  <div class="form-group" style="margin-bottom:0.25rem;">
+    <label for="error_4xx_pct">Client errors (4xx) <span style="color:var(--text-muted);font-size:0.8rem;">(% of responses)</span></label>
+    <input type="number" id="error_4xx_pct" name="error_4xx_pct" min="0" max="100" step="0.1"
+           value="{{ settings['traefik.threshold.error_4xx_pct'] }}" style="max-width:110px;margin-top:0.25rem;">
+    <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.3rem;">Above this share, a router is worth a look (amber): missing routes, failed sign-ins.</div>
+  </div>
+</div>
+{% endif %}
+
 {% macro days(label, field, key, hint) %}
 <div class="form-group" style="margin-bottom:1.1rem;">
   <label>{{ label }} <span style="color:var(--text-muted);font-size:0.8rem;">(days)</span></label>