feat(alerts): pick the host, then its part, and see what the metric reads (#5475)

#5475 "Rule form: pick the host, then its part; metrics that part reports;
a link to groups; a preview of the metric", from the operator's review of
the New rule page: hosts and their parts shared one long picker.

- Host agent rules pick a Host, then a Part: "The whole host", then its
  mounts, CPU cores, disks, interfaces and sensors under a heading each,
  cores in number order. A part that reports nothing a rule can watch
  (an interface's traffic) isn't offered.
- The Metric list holds what the picked resource reports, in catalog order.
- Under it, a preview: the reading now, and the last 24 h low to high with
  an hourly sparkline drawn from zero (UI ruling 8), read from one series.
- The group picker links to Ansible > Inventory to make a group, in a new
  tab, and re-reads the groups when the tab comes back into view.
- Picking a host or part swaps the pickers alone (/alerts/api/rule_target);
  picking a metric swaps the preview (/alerts/api/metric_preview).
- A rule's own resource and metric stay on offer after they go quiet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/templates/alerts/_metric_preview.html

@@ -0,0 +1,18 @@
+{# alerts/_metric_preview.html — what the chosen metric reads (#5475), so a
+   threshold is set against real numbers: now, and the last day's hourly
+   averages with the lowest and highest reading. A rule covering hosts
+   previews on the host picked above it, or none. #}
+{% if preview %}
+<div class="metric-preview">
+  <span>Now <strong>{{ metric_value(metric, preview.now) }}</strong>
+    <span class="tone-dim" title="{{ preview.at|when }}">{{ preview.at|ago }}</span></span>
+  {% if preview.series|length > 1 %}
+  <span class="metric-preview-day">
+    <span class="tone-dim">last 24 h</span> {{ metric_value(metric, preview.low) }} – {{ metric_value(metric, preview.high) }}
+    <span class="metric-preview-spark" aria-hidden="true">{{ preview.spark|safe }}</span>
+  </span>
+  {% endif %}
+</div>
+{% elif metric and resource %}
+<p class="field-hint">No reading of this yet from {{ resource }}.</p>
+{% endif %}