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/settings_list.html

@@ -9,7 +9,7 @@
 <p style="color:var(--text-muted);font-size:0.84rem;margin:-0.4rem 0 1.25rem;">
   Bulk agent operations across your inventory. For a single host, use its
   <a href="/hosts/">host page</a>: provisioning, a manual install command and
-  metrics live there. New hosts are added with <a href="/hosts/new">Add host</a>.
+  metrics live there. New hosts are added with <a href="/hosts/?add=1">Add host</a>.
 </p>
 
 {% if new_token %}
@@ -54,7 +54,7 @@
         </td>
       </tr>
       {% else %}
-      <tr><td colspan="7" class="empty">No agents registered yet. <a href="/hosts/new">Add a host</a>, then install its agent from the host's page.</td></tr>
+      <tr><td colspan="7" class="empty">No agents registered yet. <a href="/hosts/?add=1">Add a host</a>, then install its agent from the host's page.</td></tr>
       {% endfor %}
     </tbody>
   </table>