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>

plugins/host_agent/templates/widget_table.html

@@ -2,7 +2,7 @@
 {% if rows %}
 <div style="display:grid;gap:0.1rem;">
   {% for r in rows %}
-  {% set stale = not r.reg.last_seen_at %}
+  {% set stale = r.stale %}
   <div style="display:flex;align-items:center;gap:0.6rem;font-size:0.82rem;padding:0.3rem 0;border-bottom:1px solid var(--border);">
     <span class="dot {% if stale %}dot-dim{% elif (r.cpu_pct or 0) >= 90 or (r.mem_used_pct or 0) >= 90 or (r.disk_worst or 0) >= 90 %}dot-warn{% else %}dot-up{% endif %}"></span>
     <a href="/plugins/host_agent/{{ r.host.id }}/"