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>

tests/plugins/traefik/test_traefik_page.py

@@ -291,3 +291,25 @@ async def test_the_config_tile_says_when_it_last_reloaded(app):
                                             error_4xx=1.0, error_5xx=0.1)
     assert "Config reloaded" in page and "3m ago" in page and "343 reloads" in page
     assert "Reload failed" not in page
+
+
+# ── Error cutoffs are settings, reachable from the page (#5380) ─────────────
+
+def test_router_state_reads_the_error_cutoffs_from_settings():
+    strict = to_thresholds_cfg({"traefik.threshold.error_5xx_pct": 0.0, "traefik.threshold.error_4xx_pct": 0.2})
+    lenient = to_thresholds_cfg({"traefik.threshold.error_5xx_pct": 5.0, "traefik.threshold.error_4xx_pct": 10.0})
+    assert router_state(_latest(e5=0.05), strict)[0] == "crit"
+    assert router_state(_latest(e5=0.05), TH)[0] == "ok"           # under the 0.1% default
+    assert router_state(_latest(e5=2.5), lenient)[0] == "ok"
+    assert router_state(_latest(e4=0.5), strict)[0] == "warn"
+    assert router_state(_latest(e4=5.0), lenient)[0] == "ok"
+
+
+@pytest.mark.parametrize("role, linked", [("admin", True), ("viewer", False)])
+async def test_the_page_links_an_admin_to_the_router_thresholds(app, role, linked):
+    from quart import session
+    async with app.test_request_context("/plugins/traefik/"):
+        session["user_role"] = role
+        page = await render_plugin_template(app, "traefik", "traefik/index.html", poll_interval=30,
+                                            access_log_enabled=False, current_range="1h")
+    assert ('href="/settings/thresholds/#traefik"' in page) is linked