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/templates/settings/_tabs.html

@@ -7,7 +7,7 @@
     ("thresholds",    "Thresholds & Retention", "/settings/thresholds/"),
     ("notifications", "Notifications",  "/settings/notifications/"),
     ("reports",       "Reports",        "/settings/reports/"),
-    ("auth",          "Auth",           "/settings/auth/"),
+    ("auth",          "Authentication", "/settings/auth/"),
     ("ansible",       "Ansible",        "/settings/ansible/"),
     ("plugins",       "Plugins",        "/settings/plugins/"),
   ] %}