feat(dashboard): phase C richer panels — host time-series graph + cpu sparklines

Milestone 72 phase C — bring the host-view graphs onto the dashboard:
- host_resource_history widget reworked into a real host-view chart: epoch-ms
  linear axis (no Chart.js date adapter), themed like the host-detail charts,
  maintainAspectRatio:false so it fills the resized panel, unique canvas per
  widget instance (wid), and empty states ("pick a host" / "no metrics yet").
  Was previously unusable — it had a broken time axis and no way to choose a host.
- Add a "host" param type: the edit form renders a live dropdown of hosts
  (dashboard routes now pass the host list to the editor); the chosen host_id is
  stored in config and fed to the widget.
- Hosts-overview widget gains a per-row CPU sparkline (last hour) via the shared
  sparkline_svg helper — the host-view at-a-glance trend, on the main widget.

Charts/sparklines render only in the browser, so CI can't exercise them — needs
an operator visual check.

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

steward/templates/hosts/overview_widget.html

@@ -17,6 +17,8 @@
     {% else %}
     <span title="{{ host.name }}" style="{{ name_style }}">{{ host.name }}</span>
     {% endif %}
+    {% set spark = cpu_sparks.get(host.name) %}
+    {% if spark %}<span title="CPU — last hour" style="flex-shrink:0;line-height:0;opacity:0.85;">{{ spark | safe }}</span>{% endif %}
     <span style="display:flex;gap:0.75rem;font-variant-numeric:tabular-nums;color:var(--text-muted);font-size:0.75rem;flex-shrink:0;">
       {# Monitors #}
       {% if host.ping_enabled and ping and ping.response_time_ms is not none %}