feat(hosts): the Hosts list labels its figures and says how long; SLA tab gone; one place to add a host (#5279–#5283)

Hosts list:
- CPU and Memory are their own labelled columns (#5279); a silent
  agent's last readings are greyed, and Status says "agent silent for 7m".
- One uptime figure, 30 days, with 24h and 7d on hover. The SLA tab
  (/hosts/uptime) repeated it and is removed (#5280; one home per fact).
- Status is a word with how long ("Down for 12m"); host_status now carries
  down_since. Edit and Delete live on the host's page only (#5281).
- Figures are right-aligned with tabular digits (.table .num), not a
  monospace face.

Agent fleet (#5282): its add-host form is gone, so hosts are added only on
Hosts -> Add host. That form was also the only way to get a token for a
manual curl install, so that moves to the host's Agent card as "Install
manually" (POST /fleet/<host_id>/register), shown on the fleet page as
before. "Delete" there reads "Unregister".

Add host (#5283): laid out like the add-monitor form; a name clash comes
back inline with what was typed instead of a bare text 400; the address
is optional, as the host page already allowed.

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

tests/core/test_problems_first.py

@@ -44,12 +44,3 @@ def test_a_host_page_shows_only_the_alerts_on_that_host():
     assert not watches_host(rule("docker", "nas2/plex"), "nas", mine)
     assert watches_host(rule("http", "nas web"), "nas", mine)
     assert not watches_host(rule("http", "other web"), "nas", mine)
-
-
-def test_uptime_lists_the_worst_first_and_the_unmeasured_last():
-    from steward.hosts.service import worst_uptime_first
-    hosts = [NS(id=i, name=n) for i, n in [("a", "alpha"), ("b", "bravo"), ("c", "charlie"), ("d", "delta")]]
-    uptime = {"a": {"24h": 100.0, "7d": 99.0, "30d": 99.5},
-              "b": {"24h": 92.0, "7d": 98.0, "30d": 99.0},
-              "c": {"24h": 100.0, "7d": 95.0, "30d": 97.0}}
-    assert [h.name for h in worst_uptime_first(hosts, uptime)] == ["bravo", "charlie", "alpha", "delta"]