feat(host_agent): Netdata-style fleet overview + per-host detail view

Surfaces the metrics the agent now collects (task #868). Adds a viewer-facing
fleet page (/plugins/host_agent/) with per-host cards (CPU/mem/disk/load/temp,
stale flag) and a per-host detail page (/plugins/host_agent/<id>/) — the link
the fleet widget already pointed at but had no route for.

Detail page shows current gauges (CPU, memory incl. swap/cache, load, network
rx/tx, disk I/O, temp max, memory/cpu/io PSI), per-core CPU bars, per-mount
filesystem bars, and per-interface/disk/sensor breakdowns, plus history charts
(utilization %, throughput B/s, load & pressure) over a selectable range.
Charts use a linear epoch-ms x-axis so no Chart.js date adapter is needed.
Stale state uses the plugin's stale_after_seconds threshold. Repoints the
host_resources widget detail link from admin settings to the new fleet page.

Completes milestone #68 (task #867).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/core/widgets.py

@@ -262,7 +262,7 @@ WIDGET_REGISTRY: dict[str, dict] = {
         "label": "Host Agent — Resources",
         "description": "Fleet view: CPU, memory, disk, and load per monitored host",
         "hx_url": "/plugins/host_agent/widget",
-        "detail_url": "/plugins/host_agent/settings/",
+        "detail_url": "/plugins/host_agent/",
         "plugin": "host_agent",
         "poll": True,
         "params": [],