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/dashboard/_edit_panels.html

@@ -92,6 +92,16 @@
                   </option>
                   {% endfor %}
                 </select>
+                {% elif p.type == "host" %}
+                <select name="param_{{ p.key }}" required
+                        style="width:100%;padding:0.32rem 0.6rem;background:var(--bg);
+                               border:1px solid var(--border-mid);border-radius:4px;
+                               color:var(--text);font-size:0.85rem;">
+                  <option value="">— select a host —</option>
+                  {% for h in hosts %}
+                  <option value="{{ h.id }}">{{ h.name }}</option>
+                  {% endfor %}
+                </select>
                 {% endif %}
               </div>
               {% endfor %}