feat(dashboard): unified Hosts widget + grouped widget picker

Bring the dashboard in line with the unified host IA + improved displays.

- New core "Hosts — Overview" widget (/hosts/overview/widget): one row per host
  combining monitor status (ping dot + latency, uptime 24h) with the agent
  glance (CPU / memory / disk root + stale flag), each row linking to the host
  hub. Reads agent data from the generic PluginMetric table via a core-safe
  _agent_overview_by_host helper (no host_agent import); freshness vs the
  plugin's stale window. The granular Ping/DNS/Uptime/Agent widgets stay.
- Group the add-widget picker into Core monitors / Monitoring capabilities /
  Integrations (a `group` field on every WIDGET_REGISTRY entry + section
  headings in _edit_panels.html), matching the Settings → Plugins taxonomy.
- Fix the agent fleet widget rows to link to the host hub (/hosts/<id>) instead
  of the old /plugins/host_agent/<id>/ page.

Scribe #903.

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

plugins/host_agent/templates/widget_table.html

@@ -5,7 +5,7 @@
   {% 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 }}/"
+    <a href="/hosts/{{ r.host.id }}"
        style="flex:1;min-width:0;white-space:nowrap;overflow:hidden;text-overflow:ellipsis;font-weight:500;">
       {{ r.host.name }}
     </a>