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/core/formatting.py

@@ -229,6 +229,37 @@ SOURCES: dict[str, str] = {
 }
 
 
+def cert_left(days: float | None) -> str:
+    """Time left on a certificate: "12 days", "1 day", "today", or "expired
+    3 days ago" rather than "-3 days"."""
+    if days is None:
+        return NONE
+    whole = int(days)
+    if days < 0:
+        gone = int(-days)
+        return "expired today" if gone == 0 else f"expired {gone} day{'s' if gone != 1 else ''} ago"
+    if whole == 0:
+        return "expires today"
+    return f"{whole} day{'s' if whole != 1 else ''}"
+
+
+# Words a settings key spells in lower case that read in capitals.
+_ACRONYMS = {"url": "URL", "urls": "URLs", "ssl": "SSL", "tls": "TLS", "ip": "IP", "api": "API",
+             "dns": "DNS", "http": "HTTP", "https": "HTTPS", "snmp": "SNMP", "oid": "OID",
+             "oids": "OIDs", "id": "ID", "geoip": "GeoIP", "db": "database", "mmdb": "MMDB",
+             "wan": "WAN", "dpi": "DPI", "smtp": "SMTP"}
+
+
+def field_label(key: str) -> str:
+    """A config key as a form label: "metrics_url" → "Metrics URL",
+    "scrape_interval_seconds" → "Scrape interval seconds"."""
+    words = [_ACRONYMS.get(w.lower(), w) for w in str(key or "").replace("-", "_").split("_") if w]
+    if not words:
+        return ""
+    words[0] = words[0][:1].upper() + words[0][1:]
+    return " ".join(words)
+
+
 def metric_label(name: str) -> str:
     entry = METRICS.get(name or "")
     return entry[0] if entry else (name or "").replace("_", " ")