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>

plugins/host_agent/templates/_host_vitals.html

@@ -18,7 +18,7 @@
       {%- if reg.agent_version %}{{ " · " if not stale }}agent v{{ reg.agent_version }}{% endif -%}
     </span>
   </div>
-  <div class="stat-strip" style="margin-bottom:0;">
+  <div class="stat-strip host-health-tiles" style="margin-bottom:0;">
     {{ stat_tile("CPU", pct(cpu), state=status_class(cpu, 'cpu'), hint=sparks.cpu|safe) }}
     {{ stat_tile("Memory", pct(mem), state=status_class(mem, 'mem'), hint=sparks.mem|safe) }}
     {{ stat_tile("Disk /", pct(disk_root), state=status_class(disk_root, 'disk'), hint=sparks.disk|safe) }}