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/routes.py

@@ -690,7 +690,7 @@ async def host_vitals(host_id: str):
     # At-a-glance disk = the root filesystem (what people actually care about),
     # not the "worst" mount which is opaque. Worst stays on the full-metrics page.
     disk_root = (latest.get(host.name + ":/", {}) or {}).get("disk_used_pct")
-    sparks = {k: sparkline_svg(v) for k, v in series_raw.items()}
+    sparks = {k: sparkline_svg(v, height=24, fill_width=True) for k, v in series_raw.items()}
     # Normalize load by core count so it's comparable across hardware (load/core
     # as %). Cores derived from the per-core CPU sub-resources already collected.
     cores = sum(1 for k in latest if k.startswith(host.name + ":core"))