feat(ui): shared |bytes |rate |duration |ago filters and status_class from Settings → Thresholds (#5093)

#5093 "Shared format filters and status_class driven by Settings → Thresholds".

- steward/core/formatting.py: human_bytes (binary units), human_rate,
  human_duration (two largest units), ago (a <time> titled with the absolute
  UTC time) and status_class(value, kind, thresholds) -> ok/warn/crit/unknown,
  which raises on a kind the thresholds don't name.
- app.py registers them as the bytes, rate, duration and ago filters and the
  status_class global; threshold_style is now graded by status_class (and caps
  its weight at 500).
- Replaces the copies: docker's _human_bytes/_human_uptime, core alerts'
  _duration, the host-agent fmt_bps/fmt_bytes macros, the Traefik and UniFi
  byte-rate arithmetic, the swarm-failures ago macro and two uptime spans.
- Tests: a table per helper, and every template compiled against the app's
  filters, so an unknown filter fails CI rather than a page.

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

plugins/docker/templates/docker/_swarm_failures.html

@@ -4,12 +4,6 @@
     crash_looping, failures_1h, failures_24h and last_error; `style` is
     optional extra CSS for the wrapper. Renders nothing for a healthy service
     with no recent failures. -#}
-{%- macro ago(sec) -%}
-  {%- if sec < 90 -%}{{ sec }}s ago
-  {%- elif sec < 5400 -%}{{ sec // 60 }}m ago
-  {%- elif sec < 172800 -%}{{ sec // 3600 }}h ago
-  {%- else -%}{{ sec // 86400 }}d ago{%- endif -%}
-{%- endmacro -%}
 {%- set e = s.last_error -%}
 {%- if s.crash_looping or s.failures_24h or (e and s.state != 'healthy') %}
 <div style="display:flex;flex-wrap:wrap;align-items:baseline;gap:0.2rem 0.6rem;font-size:0.78rem;color:var(--text-muted);{{ style or '' }}">
@@ -24,7 +18,7 @@
     <span style="color:var(--text);font-family:ui-monospace,monospace;overflow-wrap:anywhere;">{{ e.error or e.message or e.state }}</span>
     {%- if e.exit_code %} · exit {{ e.exit_code }}{% endif %}
     {%- if e.node %} · {{ e.node }}{% endif %}
-    {%- if e.age_s is not none %} · {{ ago(e.age_s) }}{% endif %}
+    {%- if e.age_s is not none %} · {{ e.age_s|duration }} ago{% endif %}
   </span>
   {% endif %}
 </div>