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>

tests/core/test_formatting.py

@@ -102,3 +102,24 @@ async def test_every_template_compiles_with_the_app_filters(app):
         except jinja2.TemplateError as exc:
             failed[str(path.relative_to(_REPO))] = str(exc)
     assert not failed, failed
+
+
+@pytest.mark.parametrize("key, label", [
+    ("metrics_url", "Metrics URL"), ("scrape_interval_seconds", "Scrape interval seconds"),
+    ("geoip_db", "GeoIP database"), ("db", "Database"), ("verify_ssl", "Verify SSL"),
+    ("enabled", "Enabled"), ("", ""),
+])
+def test_a_settings_key_reads_as_a_label(key, label):
+    """#5212: plugin settings are labelled in words, not snake_case."""
+    from steward.core.formatting import field_label
+    assert field_label(key) == label
+
+
+def test_metric_names_and_units_read_in_words():
+    from steward.core.formatting import metric_label, metric_unit, source_label
+    assert metric_label("disk_used_pct_worst") == "Fullest disk" and metric_unit("disk_used_pct_worst") == "%"
+    assert metric_label("hrStorageUsed") == "hrStorageUsed" and metric_unit("hrStorageUsed") == ""
+    assert metric_label("if_in_octets") == "if in octets"
+    assert metric_unit("cert_expiry_days") == "days" and metric_unit("seconds_since_seen") == "seconds"
+    assert metric_unit("is_up") == "" and metric_unit("request_rate") == "req/s"
+    assert source_label("host_agent") == "Host agent" and source_label("my_plugin") == "my plugin"