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/core/settings.py

@@ -83,6 +83,10 @@ DEFAULTS: dict[str, Any] = {
     "ansible.max_concurrent_runs": 3,
     "ping.threshold.good_ms": 50,
     "ping.threshold.warn_ms": 200,
+    # A Traefik router serving more than this share of 5xx is failing; more
+    # than this share of 4xx is worth a look (#5380).
+    "traefik.threshold.error_5xx_pct": 0.1,
+    "traefik.threshold.error_4xx_pct": 1.0,
     # Degraded/critical cutoffs for metric coloring (warn=amber, crit=red).
     # cpu/mem/disk/load are percentages (load is load-per-core %); temp in °C.
     # uptime is "higher is better" so its warn/crit are floors. Ping latency
@@ -156,7 +160,10 @@ DEFAULTS: dict[str, Any] = {
     # Scheduled reports
     "reports.enabled": False,
     "reports.schedule_day": 6,   # 0=Monday … 6=Sunday
-    "reports.schedule_hour": 8,  # UTC hour
+    "reports.schedule_hour": 8,  # an hour in reports.schedule_tz
+    # The zone of whoever last saved the schedule (an IANA name; blank is UTC),
+    # so "Sunday 08:00" means 08:00 where they are (#5378).
+    "reports.schedule_tz": "",
     "reports.last_sent_at": "",
 }
 
@@ -452,6 +459,11 @@ def to_thresholds_cfg(settings: dict[str, Any]) -> dict:
         "tls_days": {"warn": g("thresholds.tls_warn_days", 14), "crit": g("thresholds.tls_crit_days", 3), "dir": "low"},
         "clock":   {"warn": g("thresholds.clock_warn_s", 30), "crit": g("thresholds.clock_crit_s", 300), "dir": "high"},
         "crash_loop": {"warn": g("thresholds.crash_loop_deaths", 3), "crit": g("thresholds.crash_loop_deaths", 3), "dir": "high"},
+        # One cutoff each: a router's 5xx share past it is crit, a 4xx share warn.
+        "error_5xx": {"warn": g("traefik.threshold.error_5xx_pct", 0.1),
+                      "crit": g("traefik.threshold.error_5xx_pct", 0.1), "dir": "high"},
+        "error_4xx": {"warn": g("traefik.threshold.error_4xx_pct", 1.0),
+                      "crit": g("traefik.threshold.error_4xx_pct", 1.0), "dir": "high"},
     }