feat(hosts): a host's health tiles span the card and the gaps match; Add host is a dialog on the list (#5290, #5291)

#5290 "Host page: the health tiles span the card's width; even spacing above Ansible":
the four tiles take equal quarters (halves on a phone) and their sparklines stretch
across them; the Monitors + Agent row drops the Agent card's own margin, so the gap
above Ansible is 1rem like every other.

#5291 "Add host opens as a dialog over the Hosts list": a native <dialog> on /hosts/,
opened by Add host or /hosts/?add=1. A name clash re-renders the list with it open,
the error and what was typed. GET /hosts/new is gone; hosts/form.html is Edit only.

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

tests/plugins/host_agent/test_fleet_page.py

@@ -31,8 +31,8 @@ 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)
 
-    # Hosts are added in one place, Hosts → Add host (#5282).
-    assert "fleet/add-host" not in page and 'href="/hosts/new"' in page
+    # Hosts are added in one place, the Add host dialog on Hosts (#5282, #5291).
+    assert "fleet/add-host" not in page and 'href="/hosts/?add=1"' 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