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/plugins/traefik/test_traefik_page.py

@@ -88,3 +88,25 @@ async def test_gone_routers_are_listed_by_when_they_were_last_seen(app):
     assert "1 router no longer reported</summary>" in page
     assert "old</span> <span class=\"problem-detail\">last seen <time" in page and ">2h ago</time>" in page
     assert "Failing" not in page
+
+
+@pytest.mark.parametrize("days, says", [
+    (12.7, "12 days"), (1.2, "1 day"), (0.4, "expires today"),
+    (-0.3, "expired today"), (-1.5, "expired 1 day ago"), (-12.0, "expired 12 days ago"), (None, "—"),
+])
+def test_a_certificate_past_its_date_says_expired(days, says):
+    """#5212: "expired 12 days ago", never "-12 days"."""
+    from steward.core.formatting import cert_left
+    assert cert_left(days) == says
+
+
+@pytest.mark.parametrize("enabled, says, not_says", [
+    (False, "The access log is off", "No requests in the last"),
+    (True, "No requests in the last 24h", "The access log is off"),
+])
+async def test_the_traffic_empty_state_knows_whether_the_log_is_on(app, enabled, says, not_says):
+    async with app.test_request_context("/plugins/traefik/traffic"):
+        page = await render_plugin_template(app, "traefik", "traefik/traffic.html", summary=None, history=[],
+                                            range_key="24h", access_log_enabled=enabled,
+                                            access_log_path="/var/log/traefik/access.log")
+    assert says in page and not_says not in page