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>

steward/templates/hosts/overview_widget.html

@@ -5,7 +5,7 @@
 {% from "_macros.html" import status_dot %}
 {% if not rows %}
 <div class="w-verdict" style="color:var(--text-muted);">
-  No hosts yet.{% if session.user_id %} <a href="/hosts/new">Add one →</a>{% endif %}
+  No hosts yet.{% if session.user_id %} <a href="/hosts/?add=1">Add one →</a>{% endif %}
 </div>
 {% else %}
 {% set problems = rows | selectattr("state", "in", ["down", "crit", "stale", "warn"]) | list %}