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/plugins/host_agent/test_fleet_page.py

@@ -31,9 +31,10 @@ async def test_silent_agents_come_first_and_the_table_leads_the_page(app):
                                             new_token=None, install_url=None, ansible_available=False,
                                             deploy_targets=[], deploy_groups=[], managed_key_set=False)
 
-    assert page.index("<th>Status</th>") < page.index('action="/plugins/host_agent/fleet/add-host"')
+    # Hosts are added in one place, Hosts → Add host (#5282).
+    assert "fleet/add-host" not in page and 'href="/hosts/new"' in page
     assert '<span class="badge badge-warn">Silent</span>' in page
     assert '<span class="badge badge-warn">Never reported</span>' in page
     assert '<span class="badge badge-ok">Reporting</span>' in page
     assert page.index('href="/hosts/h2">nas</a>') < page.index('href="/hosts/h1">web</a>')
-    assert 'data-confirm="Delete the agent registration for nas?' in page
+    assert 'data-confirm="Unregister the agent for nas?' in page