feat(ui): Disk, Traefik, UniFi and Settings say things plainly (#5212)

#5212 "Plain labels: no raw ids, scopes, metric keys or unexplained jargon",
batch 3:

- Disk page: each tile says what it holds. "Writable layers" becomes
  "Container files" and Build cache says it is safe to clear. Shared is
  "Shared with other images" with a tooltip, and "dangling" is gone.
  Untagged images show by id instead of "<none>", a capped list says
  "The 50 largest of N images", and each host shows "as of" its report.
- Traefik: an expired certificate reads "expired 12 days ago", not "-12
  days" (cert_left). The Traffic empty state knows whether the access log
  is on: off points to Settings → Plugins → Traefik; on says to check the
  mounted log path.
- Moments use |ago with an exact UTC tooltip: UniFi last seen, alarms,
  events, speed test and DPI; AI token last used.
- Settings: the Auth tab is "Authentication" like its breadcrumb, plugin
  fields read "Metrics URL" not metrics_url (field_label), a source's pull
  interval is "every 1h", and the last report time drops the ISO "T".

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

steward/app.py

@@ -146,7 +146,8 @@ def create_app(
     app.jinja_env.globals["status_info"] = _fmt.status_info
     app.jinja_env.globals["required"] = _fmt.required
     # A metric and its source in words, a reading in its unit (alerts pages).
-    for _name in ("metric_label", "metric_about", "metric_unit", "metric_value", "source_label"):
+    for _name in ("metric_label", "metric_about", "metric_unit", "metric_value", "source_label",
+                  "cert_left", "field_label"):
         app.jinja_env.globals[_name] = getattr(_fmt, _name)
     from .alerts.service import rule_condition as _rule_condition
     app.jinja_env.globals["rule_condition"] = _rule_condition