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/core/widgets.py

@@ -279,13 +279,20 @@ WIDGET_REGISTRY: dict[str, dict] = {
     },
     "host_resource_history": {
         "key": "host_resource_history",
-        "label": "Host Agent — History",
-        "description": "CPU/memory/disk history for one host",
+        "label": "Host Agent — History Graph",
+        "description": "CPU / memory / disk time-series graph for one host (the host-view chart, on your dashboard)",
         "hx_url": "/plugins/host_agent/widget/history",
-        "detail_url": "/plugins/host_agent/fleet/",
+        "detail_url": "/hosts/",
         "plugin": "host_agent",
         "poll": True,
         "params": [
+            # type "host" → the edit form renders a live dropdown of hosts.
+            {
+                "key": "host_id",
+                "label": "Host",
+                "type": "host",
+                "default": "",
+            },
             {
                 "key": "hours",
                 "label": "Time range",