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/integration/test_monitors.py

@@ -125,3 +125,5 @@ def test_last_results_are_the_newest_n_oldest_first_and_host_status_reads_the_la
     assert len(last[b]) == 1 and last[quiet] == []
     assert status[hid]["state"] == "down" and status[hid]["count"] == 2
     assert status[hid]["latency_ms"] == 12.0
+    # Down since a's first failure after its last success (#5281).
+    assert abs((status[hid]["down_since"] - (now - timedelta(minutes=1))).total_seconds()) < 1